IMHO, most people using SIP providers are going to hook it up to something that feels like a traditional phone. Maybe that's a ATA that provides a FXS port to plug in a regular or cordless POTS phone, or as the backend for deskphones for a business. I've been using voip.ms for a long time for this; despite the domain, I think they're based in Canada. I think they got some investment recently and they've rebuilt their website to be more flash and less substance, but service continues as-is, at least for my minimal usage (except for the time they and a lot of other voip providers were getting DDoSed; that was kind of rough).
They've got a wiki about using their service from a smartphone [1], but I'm guessing it's fairly dated; I don't think Android has a sip client integrated into the dialer anymore? And with Doze and modern networking, I don't think you're going to have a good time with a plain SIP client, if you want to get unexpected inbound calls; and for iOS, you always needed a server assist to get a push notice for unexpected inbound calls. I don't know if a regular SIP service is going to let you roam from wifi to cell and vice versa during a call either. If you want things to work well, you really need an intermediate server built around how modern mobile apps can communicate ... and then you get awful close to just using one of the OTT messengers and whatever calling in that app.
Voip.ms also has a more recently published guide [2] in their blog... so I dunno, maybe? Vonage claims to have an app for android/ios for their business communications customers [3] that might work, too?
They've got a wiki about using their service from a smartphone [1], but I'm guessing it's fairly dated; I don't think Android has a sip client integrated into the dialer anymore? And with Doze and modern networking, I don't think you're going to have a good time with a plain SIP client, if you want to get unexpected inbound calls; and for iOS, you always needed a server assist to get a push notice for unexpected inbound calls. I don't know if a regular SIP service is going to let you roam from wifi to cell and vice versa during a call either. If you want things to work well, you really need an intermediate server built around how modern mobile apps can communicate ... and then you get awful close to just using one of the OTT messengers and whatever calling in that app.
Voip.ms also has a more recently published guide [2] in their blog... so I dunno, maybe? Vonage claims to have an app for android/ios for their business communications customers [3] that might work, too?
[1] https://wiki.voip.ms/article/Android
[2] https://blog.voip.ms/blog/voip-on-cell-phones-ios-and-androi...
[3] https://www.vonage.com/downloads/