I am having a problem with calling with SIP (JMP.chat) on the Librem 5. When I dial a number using my SIP service, it dials and calls just fine. However, if I am ever asked to dial something on the dialpad, I can no longer do it. The dialpad “appears” below the call but it does not function. It does not appear to have any underling plugin(?) to make the dialpad arise. I cannot access the dialpad any longer and therefore I am stuck never being able to complete a call that requires I make a dialpad choice. (For example, choose number “1” to speak with the operator, choose number “2” to dial by directory, etc).
DTMF can work with VoIP (refer RFC 2833 or RFC 2976, for example) but obviously it depends on compatible support at both ends (or even also in intermediate parties).
You may get away with DTMF inband, providing that you are using a high quality codec.
So one way or another the VoIP client may need configurability in order to support a suitable option.
And, yes, the VoIP client needs the smarts to make the dialpad available otherwise it’s all a non-starter.
(If using the inband option, would it work to generate the tones separately from the VoIP client? LOL.)
I just heard of VOIP, SIP, and the Calls app as a GNU alternative of making calls over the IP address or internet. I could practically save budget by trimming call services and investing in data services, since data services can help make phone calls. I am not sure if a phone number is assigned to the registered VOIP account, but it would make more sense that way.