1. get a phone, try to plug it in see if it works
2. ask the project manager of the construction
3. or ask the landlord
4. Walk around the house (outside your apartment) to find a telephone cabinet/box. This is where all lines merge. Hope there is a sticker on the wires with the phone number.
5. If your phoneline is dead, call swisscom and ask for a new line (ANALOG! no ISDN b.s.!). Give them your name, address, and maybe the floor/door numbers. They will find you.