I am confuse now.The way that I understood the question is:
1)Customer call into the Cisco 5300 using PSTN lines,carried over T1 circuits provided by your friendly phone company.
2)Those T1 lines are connected to the T1 ports of the Cisco 5300.
3)Calls are answered by the Cisco box, customer get a prompt, enter account number and pin #, dials the remote number.
4) Call is packetized by the Cisco, converted to VOIP format and sent to the other end directly or via a ITSP (Internet Telephony Service Provider).
On this configuration the call enter the Cisco box on normal telephony format (64 KB per call, one channel per user, 23 or 24 calls per T1 depending on signaling type) and come out of the box over the ethernet port as packets of data. The bandwith used will depend on the codex used, but a rule of thumb will be around 12-15 kb of bandwith for each call if you want excellent quality.
If you want to cut corners and offer low quality you could get away with as low as 6 kb per call.
The answer to the original question is : You could get up to four T1 of traffic from the phone company on a full T1 (1.544 mb) of data on your ethernet port with very good quality.
That is a four to one ratio. If you go cheap you could do up to eight to one.
I have a woorking gateway (Quintum) that is running like a champ with four channels on a 64 kb circuit.