the best they (and any other ISP) can do is to sign you up and process the order. then some point later the order will come back with a decline because the ports are all full.
then you just have to wait for somoene else to move house or cancel their broadband and you can be in (assuming you are first in the queue, you might not be)
It's not a very nice situation to be in, but there isn't much you can do about it. Whichever ISP you signed up with would have signed you up, then not been able to connect you (except possibly a UCLL one if you are in an unbundled exchange)