With Nextcloud Talk you can do audio/video calls inside your system.
If you want to call traditional phones you need “VOIP” and phone numbers. This integration was briefly shown in the last “Talk Munich” but I’m not aware of any public docs now - see NextCloud Talk Phone (SIP) for discussion.
SIP trip phone is a “VOIP client” running inside of Nextcloud (which makes more or less no sense in my eyes.. but who knows) you still need either a PBX or public provider offering you the connection with the phone network.