If your business still runs an on-site phone system on ISDN or analogue lines, you have a decision to make before Openreach switches the old network off on 31 January 2027. The two most common routes are a SIP trunk, which keeps your current phone system and swaps the lines underneath it, or a hosted PBX, which moves the whole phone system into the cloud. This guide to SIP trunk vs hosted PBX explains the difference in plain English and helps you work out which suits a business of 10 to 50 people.
What is a SIP trunk?
A SIP trunk is a phone line delivered over your internet connection instead of copper. It plugs into the phone system you already own (the box in the comms cupboard, often called a PBX) and does the job your ISDN or analogue lines used to do.
SIP trunks are usually sold by the channel. One channel carries one call at a time, so you buy as many channels as the number of calls you expect to be on at once, not one per handset. As a rough guide, a ten-person office typically needs three or four channels.
Most modern systems work with SIP, including 3CX, FreePBX, Asterisk, Avaya, Mitel, Panasonic and NEC. Very old systems that only accept traditional lines may need a gateway adapter, or it may make more sense to replace them. Our SIP trunking page has more detail.
What is a hosted PBX?
A hosted PBX (also called a cloud phone system or hosted VoIP) moves the phone system itself off your premises. The system runs in a data centre and your staff connect to it with desk phones, a desktop app or a mobile app. There is no box in the cupboard to maintain, and calls reach people wherever they are working.
A hosted PBX is normally priced per user per month, and the phone system features (call queues, ring groups, voicemail, an auto attendant, call recording and so on) are included in that price. Read more on our hosted PBX page.
SIP trunk vs hosted PBX: the key differences
Where the phone system lives
- SIP trunk: your PBX stays on site. You (or your maintainer) still look after the hardware, software updates and repairs.
- Hosted PBX: the system is in the cloud and your provider manages it. Your office only needs a reliable internet connection and the phones.
How you pay
- SIP trunk: per channel, plus call charges or a call bundle, depending on the provider. You also keep paying for any maintenance contract on your existing PBX.
- Hosted PBX: per user per month. Our plans start from £14.99 per user per month, with higher tiers adding softphone apps, SMS, Microsoft Teams integration and call centre features. For a fuller breakdown, see our guide to how much a business phone system costs in the UK.
Upfront cost
A SIP trunk is usually the cheaper way to get off ISDN quickly if your current phone system is in good shape, because you are not buying a new system. A hosted PBX may involve new handsets (or none, if your team uses the apps) and a setup fee, but it removes the on-site hardware you would otherwise have to replace one day.
Remote and mobile working
Some on-site systems can support remote users, but it often means extra licences or configuration. With a hosted PBX, working from home or on the move is built in: staff can make and take calls from your business number on their mobile, laptop or desk phone. See how our mobile and desktop apps work.
Resilience
Both options depend on your internet connection, so plan for what happens if it drops. With a SIP trunk, calls can be set to divert to mobiles or another site if your connection fails. With a hosted PBX, the system itself keeps running in the data centre, so calls can still reach staff on the mobile app even if the office connection is down.
Keeping your numbers
Either way, you can keep your existing business numbers. Number transfer is free with us, and your numbers stay live during the move.
When a SIP trunk makes sense
- Your current PBX is fairly recent, supports SIP and still does everything you need.
- You have recently invested in the system or handsets and want to get full value from them.
- Most of your team works in the office and you are happy to keep maintaining hardware on site.
- You need to get off ISDN or analogue lines quickly with as little change for staff as possible.
When a hosted PBX makes more sense
- Your phone system is old, out of support or expensive to maintain.
- Staff work from home, travel or split time between sites.
- You want one predictable monthly cost per user rather than separate line, maintenance and hardware bills.
- You want features such as call queues, call recording, Teams integration or website live chat without buying add-on hardware.
- You would rather your provider handled updates and security than an in-house box.
Many businesses use the switch-off as the moment to move to the cloud, because they would need to replace an ageing system before long anyway. If that is you, our phone system migration service covers the move, including number transfer and setting up your call flows.
Can you do both?
Yes. A common middle route is to move to SIP trunks now to get ahead of the switch-off, then move to a hosted system later when the old PBX reaches the end of its life. Hosted platforms such as hosted 3CX can also use SIP trunks behind the scenes, so the numbers you move today can follow you.
If you are still on ISDN, our guide to ISDN replacement options walks through every route in more detail.
Questions to ask before you decide
- How old is our current phone system, and is it still supported by the manufacturer?
- How many calls are we on at the same time during our busiest hour?
- How many staff work away from the office at least part of the week?
- What are we paying today for lines, calls and maintenance combined?
- Is our internet connection good enough for voice, and do we have a backup?
- Do any other devices, such as alarms, lift lines or card machines, use our phone lines?
If you are not sure about any of these, our free PSTN switch-off check will list what is connected to your lines and what needs to change before January 2027. You can find out more on our PSTN switch-off page.
Not sure whether a SIP trunk or hosted PBX is right for you?
Tell us about your current phone system and we will recommend the simplest route off ISDN or analogue lines, with free number transfer so you keep your numbers.