[calmpitbull]

A third wire is not needed for a master socket, only for an extension socket. Plus in my experience the dect cordless phones don't even use the third (bell) wire as they have a built in capacitor which fakes a master socket.

When you say the phone only rings twice, does the callers ring tone also stop or does it carry on?

I suspect the phone, first try a cheap corded phone before you do anything else.

If the phones intermittently rings then it cannot be a failed number port, if it was a failed port then the phone would never ring.

If the second tech told you that his test phone was ok, I suspect the phone even more. Change it for a cheap corded one for a few days, if the fault disappears, get your cordless ones replaced.