Question about Gigaset C530 Ip

Here you can find the instructions for the configuration ("Configuring the SIP telephone")
https://www.swisscom.ch/de...
It worked for me without any problems.

Yes, the telephone can also be used as an analogue (no ISDN) landline telephone. This is also possible together with VoIP (internet telephony). It does not matter whether it is Swisscom or another fixed network telephone provider.