Since the end of April, Swisscom has been communicating the access data for telephony (VoIP) to its private customers.
The procedure is explained in rudimentary form at the address:
https://www.swisscom.ch/de....
Official NATEL login required at Swisscom. You need this for the two-factor authentication, because the data is transmitted by SMS.
Select "Manage SIP Credentials". After the second login, you will be asked to enter the code. This completes the connection of your number. Now you can create the SIP credentials and have them sent to you by SMS.