I don’t think there’s a ready-made solution for something like this. If you were going with the coding route and we’re just concerned about making calls and not receiving them on your main phone, then you could verify your second number with a SIP carrier like twilio so that you can use it as an outgoing caller ID with them, create a sip trunk with them, spin up a local asterisk server and hook it up to the sip trunk, and then program your asterisk dialplan to allow you to dial out using that second number as the caller ID. Then you’d just use an app like linphone, sign in using your asterisk credentials, and dial out from there. It’s a lot of setup to do if you’re not familiar with asterisk or sip trunking though