Is there a simple way to use mobiles as SIP endpoints?

It strikes me that it ought to be easy to use a mobile OS to run the software that's embedded in 'SIP phones'.

I've not checked the protocol yet, but, presumably, at least for IPv6, the big gap is the mapping between a phone number and the corresponding IP/port address pairs (I'm guessing a bit, but that's the obvious place to assert some sort of commercial wedge, otherwise phone numbers would just be like DNS names and we could have as many as we wanted, virtually for free). [For IPv4, I'd guess that there's something nasty like STUN/ICE to establish the p2p link across intervening NATs]

Are there any suitable setups/approaches?

You can download various SIP phone apps from the App Store - e.g. https://apps.apple.com/gb/app/bria-mobile-voip-softp... is just one of many - but I don’t think iOS or Android have any built in SIP functionality

A&A also offer SIP2SIM, which allows your SIM card to behave like any other VoIP extension in the office, so things like call recording, internal extensions, hunt groups, queues, etc work. The difference with this is that the VoIP stuff is done remotely by A&A, rather than on your phone. https://www.aa.net.uk/voice-and-mobile/sip2sim/

So A&A's assertion is that you cannot get a reliable service with something like the Bria app. SIP2SIM turns your mobile phone into a.... phone - connected to the pstn (via the mobile network).

I'm sure that they know what they're talking about - they have a good reputation - but I'm surprised that you can't turn the Yealink into s/w. 120gbp for a phone seems like a lot to me.

I don't think that there's anything special about SIP - it's defined in various RFCs, so it should be easy enough to get it onto Android (ie Linux). You do need the service to provide the bridge from SIP based VoIP to the PSTN world of phone numbers.

In reply to a post by timcoote:

So A&A's assertion is that you cannot get a reliable service with something like the Bria app.

so this could be true... The Bria app does stuff where they can register your SIP account in the cloud, and use that to send push notifications to your device when you receive a call. I guess this is to make work around problems with apps being killed in the background if they're not in active use.

And yes, there's nothing special about SIP. It's an open standard. The only complication I can think of is that SIP isn't particularly designed with low data / low battery requirements, and I'm not sure how well it would work on a mobile phone behind CGNAT, or with apps that are limited in how much processor power they can use in the background.

Is this what you're trying to achieve? Linky

In reply to a post by timcoote:

So A&A's assertion is that you cannot get a reliable service with something like the Bria app.

They want to sell you their own Broadband service with a set of IP addresses, and each SIP device has its own public IP. Whilst I’m a happy customer of A&A’s VoIP service, I use with a mobile app, I don’t disagree there is a ‘professional’ way for a business, and an ‘acceptable’ way for home use.

I can’t get A&A’s broadband at any decent speed, so I don’t use it.

There could be issues with CGNAT, but I'm only thinking if using it at home and I can use IPv6 for that. Good point about power consumption.

I think so. I'll have a closer look.

I can see this space getting messy as PSTN stop sell is next month

I have the Grandsream GS Wave app installed on my Android phone, and registered to an A&A VOIP account.

AFAIK, The difference between A&A's SIP2SIM service and running a SIP client app on a mobile is that, from the underlyiing mobile network point of view, calls over SIP2SIM are treated and prioritised as any normal voice call, whereas a SIP call is competing for data bandwidth.

The distinction is important when you are connected to a cell with heavy data contention e.g. at a large event in a rural area - you may find that you have good signal coverage but little to no data throughput, but the network is still able to deliver "voice" calls.