A good rule of thumb comes from old telephony .. an phone T1 line 24 voice channels and a control channel. This equates to roughly 64Kb/s per phone line. This rule works (roughly) for uncompressed VoIP. There are various codecs that do compression .. the G729 should roughly triple that, so get 72 voice channels on a T1.
If you expect all 55 phones to be on external calls at the same time, you will utilize roughly 75% of a T1. That is pushing the realistic limits. A single T1 will be fine if you expect less than 80% or so phone utilization. Generally speaking, as you push the bandwidth limits you will see degraded quality before you see calls fail, so a T1 should be OK for the occasional peak to 100%.
IMHO the kind of connection matters little, as long as you get low latency between between the phone and the "switch" (aka the VoIP server).
Were I in your shoes I would get a T1 and watch the performance, call quality, the number of phones, and the average phone utilization. I would also be careful about compression .. it can really do a number on call quality, particularly when combined with tight bandwidth.
Also, if you are going to put phones and PCs on the same network, make sure you get QoS running properly on the internal switches and the internet router. It would suck to have enough bandwidth but have poor call quality because of the call center reps watching videos on YouTube or opening files on the server. If you don't know how to deal with QoS, either get someone who does, or put the phones and computers on separate networks and use separate internet connections.