You need to be connected to your internet box WLAN at home to use the telephony function.
You also need a Swisscom Internet subscription with IP fixed network telephony.
If the telephony function is not available in your home network either, deactivate the function in “Telephone” > “Settings” and perform a reset by reactivating the function. If this does not work either, log out and log back in again, reinstall the app and reboot the Internet-Box (in this order).