If customers say they ignore your calls because they look like spam, or you’ve seen warning labels on your outbound number, the cause is usually one of a few things: how your number behaves, how it’s presented and who is judging it. The good news is that most of these are fixable. The bad news is that not all of them are in your provider’s control.

The problem is growing more visible as businesses move to digital lines. The UK’s legacy phone network is due to be retired on 31 January 2027, and Openreach has said it will not be delayed again. Firms replacing ISDN with a SIP trunk for business use often find that their outbound calls suddenly look different to the people receiving them.

This guide explains what’s going on and what to do about it.

Who Decides a Call Is Spam?

Not your SIP provider, and not Ofcom. Spam and scam labels usually come from the receiving end:

• Mobile network tools that flag suspicious numbers

• Call-blocking and caller-identification apps installed by the recipient

• Handset features that screen unknown callers

• Analytics services that score numbers based on observed behaviour

These systems don’t publish their rules, and they don’t all agree. A number might be flagged by one app and look perfectly normal on another handset. That’s why fixing the problem is partly detective work.

Common Causes

Calling Patterns That Look Automated

Analytics tools look at behaviour. A number that makes lots of calls in a short time, many of them very short or unanswered, looks like a robocaller whether it is one or not. Sales teams with dialling software, appointment-reminder systems and busy contact centres can trigger this without meaning to.

A Number With No History

A newly activated number has no track record. Making hundreds of outbound calls from a fresh number on day one can look suspicious, because that’s exactly what scammers do.

A Recycled or Previously Abused Number

If your number was used by someone else before you, you may inherit its reputation. It’s a common reason a newly ported or newly issued number is flagged from the start.

Changing or Rotating Numbers

Some businesses rotate through numbers to avoid being blocked. That tends to backfire, because it removes any reputation you’ve built and resembles the behaviour of spam operations.

A Presentation Number That Doesn’t Match the Caller

Your outbound calls carry a number the recipient sees, called the presentation number. If it’s one that doesn’t ring back to your business, isn’t one you’re entitled to use or doesn’t match who is actually calling, it looks wrong to both people and systems.

Calls Routed Through Odd Paths

Calls that pass through several networks, or that originate abroad and present a UK number, are more likely to be treated as suspect.

Complaints and Low Answer Rates

If recipients report your number, or if most calls go unanswered, the score tends to fall. A low answer rate can happen if people simply don’t recognise the number.

What the UK Rules Say About Caller ID

In the UK, caller identification is governed by Ofcom’s General Conditions, and Ofcom’s CLI guidance sets out how providers must handle it, alongside the Privacy and Electronic Communications Regulations. 

The core requirement is that, where caller information is provided, it must include a valid, dialable telephone number which uniquely identifies the caller. The guidance distinguishes the network number, which identifies where the call came from, from the presentation number, which is what the recipient sees. Businesses can legitimately present a different number, such as a main switchboard, provided it is valid, dialable and one they’re entitled to use.

Ofcom has also been tightening rules on spoofing. From January 2025, its guidance has said providers are expected to identify and block calls from abroad that present a UK number, except in a limited number of legitimate cases. Ofcom published a further statement on tackling scam calls from abroad and updated its CLI guidance in July 2026. Rules in this area are changing, so check the latest guidance or ask your provider how it applies to you.

One practical consequence: if you have staff or offices abroad making calls with UK numbers, expect closer scrutiny. Talk to your provider before problems arise.

If you make marketing calls, additional rules apply, including requirements on identifying yourself and giving people a number to call back, so check the Information Commissioner’s Office guidance too.

What Your SIP Provider Controls, and What It Doesn’t

Be realistic about this when you speak to them.

Your provider can: ensure your trunk sends valid caller information, configure the presentation number correctly, tell you how your calls are routed, flag unusual traffic, advise on dialler behaviour and support you if a number is mislabelled.

Your provider can’t: override a third-party app’s judgement, force a mobile network to change a label or guarantee that no recipient will ever see a warning.

A good provider will tell you which is which, and will help you work through the parts that are within reach.

How to Diagnose the Problem

1. Test the number. Call it from several handsets on different networks, with different call-blocking tools if possible. See what each shows.

2. Check the presentation number. Confirm it’s valid, dialable, owned by you and one that rings back to your business.

3. Look at your calling pattern. How many calls per hour, how long, how many unanswered?

4. Check the number’s history. Was it recently ported or issued? Could it have previous owners?

5. Review your dialler or CRM settings. Some systems cycle through numbers automatically or dial in rapid bursts.

6. Check for complaints. Ask staff whether customers have mentioned the problem or reported the number.

7. Ask your provider to review call records for oddities, such as unexpected routing or international legs.

How to Fix It

Use consistent, genuine numbers. Present a valid number that customers can ring back and that reaches someone who knows what the call was about.

Warm up new numbers. Start with modest volumes and build up over time, instead of making hundreds of calls on day one.

Slow and shape your dialling. Avoid rapid bursts, very short calls and repeated redials of unanswered numbers.

Answer return calls. If people ring back and nobody picks up, that reinforces the suspicious pattern.

Don’t rotate numbers. A stable number builds a reputation. A revolving door destroys it.

Prime your customers. Where appropriate, text or email ahead to say when you’ll call and from which number.

Check your lists. Calling people who haven’t expected a call, or have opted out, generates complaints that hurt your reputation.

Report mislabelling. Some call-blocking apps and analytics services offer a way to challenge a label. Your provider may be able to advise which routes are available.

None of this guarantees results, but together these steps address the usual causes.

Questions to Ask SIP Providers

When comparing SIP trunk providers in the UK, add this topic to your list.

• How do you handle caller information and presentation numbers?

• Do you check that I’m entitled to use the number I present?

• How do you deal with calls from users or offices abroad that present UK numbers?

• Do you monitor outbound traffic for unusual patterns, and will you tell me?

• What support do you offer if one of my numbers is mislabelled?

• How are numbers allocated, and could I receive one with a poor history?

• Do you support any technical call-authentication measures, and what are your plans?

• What happens to my caller ID if calls fail over to another route?

Reading the Market

You’ll find plenty of lists of the best SIP providers in the UK or the best UK VoIP providers online, and plenty of searches for the cheapest UK VoIP providers. Few of them say anything about how a provider treats caller identification, number quality or support when labelling goes wrong. A low channel price may come with generic routing, little monitoring and no help with reputation problems.

If outbound reputation matters to your business, ask each provider direct questions about their IP SIP trunk routing, number sourcing and support, and weigh the answers alongside price.

A Realistic Example

A 15-person debt advice charity in Manchester switches from ISDN to a SIP trunk. Within weeks, advisers report that clients aren’t answering, and several say the calls show as suspected spam.

After investigating, they find three issues: the charity’s caller ID shows an individual direct number that rings nowhere, the dialling software redials unanswered numbers within minutes and the new numbers have no history. They change the presentation number to the main switchboard, which is staffed, spread the dialling over longer intervals and begin with a small volume of calls on the new numbers. They also text clients ahead of appointments.

Within a few weeks, answer rates improve. The charity also asks its provider to review call records quarterly.

A Simple Prevention Checklist

1. Present a valid number that rings back to a real person

2. Keep numbers stable and avoid rotating them

3. Ramp up volumes gradually on new numbers

4. Avoid bursts, rapid redials and very short calls

5. Tell customers when and from where you’ll call

6. Review call patterns and complaints regularly

7. Choose a provider that engages on caller ID and reputation

Wavetel Business is one example of a UK provider you might speak to about this, and any provider offering a SIP trunk service should be willing to explain in writing how it handles caller identification and what support it gives if a number is mislabelled.

Frequently Asked Questions

Why do my business calls show as spam?

Usually because of calling patterns that look automated, a number with no history or a poor one, a presentation number that doesn’t match the caller or complaints from recipients. The labels come from the receiving side, not from your provider.

Can my SIP provider remove a spam label?

Not directly. They can check your configuration, review your traffic and advise on how to challenge a label, but they can’t control third-party apps or mobile network tools.

Does Ofcom regulate spam labelling?

Ofcom regulates caller identification and requires providers to handle it properly, including blocking certain spoofed calls from abroad. Spam labelling by apps and handsets is separate, and Ofcom doesn’t control it.

Can I show my main switchboard number on outbound calls?

Generally yes. The presentation number must be valid, dialable and one you’re entitled to use, and it should identify your business. Confirm the setup with your provider.

Will changing numbers fix the problem?

Usually not. A new number has no reputation, and frequent changes can make you look more suspicious. It’s better to diagnose the cause and fix your calling behaviour.

Conclusion

Calls get marked as spam when their behaviour or presentation looks like a scammer’s, and the judgement is made by apps and networks your provider doesn’t control. The most common causes are bursty dialling, new or recycled numbers, mismatched caller ID and unanswered return calls.

The practical takeaway is to test your numbers, fix your presentation number and calling pattern and ask your provider direct questions about caller identification and support. Do it early in your move to SIP, well before January 2027, so you can build a good reputation before your old lines disappear.