waynesa98 recommended VOIP cable modem. That would introduce network reliability issues for your phone system. When any node in the series of nodes you connect through to central office goes down, every node after that one goes down. i.e., your VOIP is also down.
We have been assuming you only have one RJ45 port at each station.
We did a network upgrade with a phone system a few months ago. We found it cheaper to through an inexpensive switch in at a number of stations that adding additional ports and cabling. Adding an inexpensive switch would solve your problem of the 1gigabit through 100megbit limitation.