There are 23 channels in a standard T1. If you have 2 T1s bonded together your maximum available bandwidth is 3 MB/sec (1.5 per T1).
A CO is the telephone companies Central Office.
I believe there are 24 channels in Robbed Bit T1. There are 23 channels in ISDN PRI (T1). Speed can be throttled from your ISP/Telco/CO. Find out what your protocol is to verify what you should be getting.