shamelessly advertising SIP support in gnome-calls 
If you want to use it for PSTN calls you (currently) need to dial a telephone number as a SIP address:
$NUMBER@your_sip_provider.tld
jmp.chat worked nicely when I last tried.

If you want to use it for PSTN calls you (currently) need to dial a telephone number as a SIP address:
$NUMBER@your_sip_provider.tld
jmp.chat worked nicely when I last tried.