Buy or borrow a phone extension lead and have it available when they install the remote box. Tell them that your friend that works in BT is coming around next Thursday to permanently wire it up properly*. When it's all running, remove lead and put it aside just in case they call you up and want to know why it's not connected to the phone line. Generally if the main box is attached to the phone line, they won't get too upset about the others, but you have signed a contract and this would technically be breaking it!
And no, I don't believe you can have a wireless phone connection in the manner you're talking about. They would have been popular back when everyone was using dial-up but I haven't seen anything like it.
* not true probably