On my Android, I use ZoiPer (although I keep meaning to switch to using Android’s built in functionality - go to the “Dialler/Telephone”, select the 3 dot menu top right to access Settings, select Calls and then Calling accounts).
On Windows, I use ZoiPer5 (I used to use Counterpath’s Bria: but that ended up being more and more expensive with them discontinuing/forcibly blocking old versions).
We did also have a dual wireless phone system which supported standard landline and SIP connections (VoIP) - but I can’t seem to find which brand that was. Some network routers (like our Draytek Vigor2860Vn+) support having an inbound PSTN (normal phone line) connection, external SIP accounts and normal “internal” phone connectors - but I haven’t tried that setup yet.
Our VoIP accounts are with Gradwell, but we’ve also got a Twilio number forwarding to them. Gradwell certainly is not the cheapest, but the friction involved in porting numbers and the effort I’m currently willing to put in don’t correspond.
If Zevvle could offer a “high-tech” solution - such as you pick a number (01, 02, 03, 07 etc), it’s allocated to your Zevvle account and then any phone calls or text messages are then routed as you want (to your Zevvle SIM whilst calling your desktop SIP phone and then falling back to your designated landline after 4 rings, with text messages going to the SIM and email) then wowsers!