Hello Mona
The Cisco SPA 303 is a VoIP phone. You cannot plug it directly into the wall. It has to be connected with an Ethernet cable to a router or simply to a network. Then it has to be configured with a SIP account. I don't know if UPC offers this. You can easily get this SIP data from Swisscom, for example. But you can also create a SIP account with another provider. It's similar to the e-mail address. You can be a customer of UPC, but have an e-mail from gmx, for example.
The ports on your UPC modem are for "normal" analogue phones.
I hope I didn't clarify too much ;)
Greetings Michi