Normally

The phone installations are already there.

You just need to know who was the last tenant who lived on that appartment and had a fixed telephone line.

So then Swisscom can find out the circuit number and do the connection at the Phone Central.

If the previous tenant didn't had a fixed line ( could be the case, there are some people now that only need cell phones ), then you need to find out who was there before him. Ask the administration company ( "La Regie" ) for the list of the last tenants.

Other possibility is that a previous tenant could have had an ISDN connection installed. In that case the cables need to be reconnected back to the analog line mode.

Only on that case you'd need an electrician.