Choosing a SIP Trunk Provider and Getting Memorable Local Numbers

By John Rowlinson

If you run your own phone system, such as 3CX or a similar IP PBX, you still need two things before anyone can ring you: a connection to the public phone network, and some phone numbers. Both usually come from a SIP trunk provider, and the choice affects your call costs, the numbers you can offer customers and how exposed you are to fraud.

SIP trunks and DDI numbers in plain terms

A SIP trunk is the connection that carries calls between your phone system and the wider phone network. Your phone system handles the internal side: extensions, ring groups, voicemail and queues. The trunk carries calls out to landlines and mobiles and brings incoming calls back in.

A DDI (direct dial in) number is a phone number on that trunk which routes straight to a particular place in your system, such as one person's desk phone, a sales queue or an out of hours message. You can have many DDIs on one trunk, and your phone system decides where each goes.

Your phone system and your trunk provider do not have to be the same company. You can choose a provider that is compatible with your PBX and permitted by your hosting arrangement; some managed 3CX hosting limits which trunks you can use, so check first. That separation gives you choice. If a provider raises prices or its service slips, you can add another trunk or move your numbers without rebuilding the phone system.

What to compare between trunk providers

• Number choice in your area code: can you search and pick local numbers, or are you simply allocated one?
• Total cost: number rental, call charges and channel limits are separate things. How many numbers you have does not decide how many calls you can handle at once. Also check VAT, setup and porting fees, minimum charges and contract length.
• UK and international call rates: compare rates to landlines and mobiles, and whether any bundle fits how much you actually call.
• 3CX support: check the current list of supported SIP trunk providers on the 3CX website. Supported providers usually come with configuration templates.
• Number porting: can you bring existing numbers in, and take them out later?
• Emergency calls: confirm 999 and 112 work and register the address where phones are used. Ask how the provider handles staff working from home or several sites, since one office address may be wrong for a home worker, and which outgoing number identifies each location.
• Resilience: internet calls depend on power and broadband. Ask about diverting calls to mobiles when your system is unreachable, and consider backup connectivity and a UPS. A second trunk over the same failed connection does not help.
• Support hours, time zone and billing currency: a provider abroad may be closed when you need it, and paying in dollars or euros means costs move with exchange rates.
• Paperwork: expect to provide identity and address documents. Check the provider's eligibility requirements for UK geographic numbers before ordering; Ofcom publishes the numbering rules.
• Data protection: call logs, voicemail and recordings are personal data. Check the provider's data processing terms, retention, access controls and any overseas transfers. The ICO has guidance for small businesses.

Fraud controls deserve their own check

Toll fraud is the main risk with any internet phone system. If someone gets into your phone system or steals your SIP credentials, they can route calls to expensive international or premium rate numbers, often overnight or at weekends, and the bill can be large by Monday morning. Depending on your provider's terms, you may be liable for those calls.

Ask what the provider offers, and switch it on:

• Spending limits or a prepaid balance. Ask whether caps are genuinely hard limits, how quickly they act and whether charges can exceed them.
• Blocking international and premium rate destinations you never call.
• Long, randomly generated SIP credentials, and multi-factor authentication on your provider and PBX admin accounts.
• Restricting the trunk to your phone system's IP address where supported, as an extra layer rather than a substitute.

Limit who can reach the PBX management interface and apply updates promptly. The NCSC's Small Business Guide is a good starting point. If you suspect fraud, contact the provider immediately, block the compromised access, keep the logs, and report it to the national fraud reporting service for England, Wales and Northern Ireland, or to Police Scotland.

Your situation may be slightly different. ask a question below ↓ and our editorial team will reply with our advice.

Using several numbers, and making them memorable

Additional numbers can help where they serve a clear purpose, though each one needs routing and upkeep:

• Separate numbers for sales and support, so calls reach the right queue.
• A direct number for each member of staff.
• Memorable sequences that customers recall without looking them up.

Number selection varies between providers. For my own SIP trunks I use Telnyx, chosen for its choice of local numbers: I could pick from a big range in my own area code for about a dollar each, and the line costs me about a dollar a month. I got memorable sequences such as 411000, 411411, 411001 and 411002 on my local 01 code without paying any premium. When I wanted a memorable number from Voipfone in the UK, I paid for a premium number.

Those prices are from my own experience at the time, in US dollars, and they change. Check current rates with any provider before deciding.

Porting your existing numbers

If customers know your current number, you will probably want to keep it. Moving it to another provider is called porting:

1. Check your current contract for notice periods and minimum terms.
2. Ask the new provider whether it can port your type of number and what it needs. The account name and address must match the old provider's records.
3. If the number is on a traditional landline, ask whether porting will affect broadband or other services on that line.
4. Set up the new trunk and test it with a temporary number.
5. Submit the port request and wait for a confirmed date.
6. Keep the old service live until the port completes. Cancelling early can lose the number.
7. Afterwards, test incoming calls from more than one network, check your outgoing caller ID, and confirm the emergency address records with the provider rather than making a test call to 999.
8. Confirm which services the port has ended, and cancel only those remaining services you no longer need.

Ports can take days or weeks, so avoid scheduling one during your busiest period.