There are differenct types of T1. Depending on type of Voice and data. All T1 is 1.544Mbps but the payload changes depending on signalling.
T1 with ESF B8ZS will deliver anything. and has 1.536Mbps payload or if it is ISDN 23 X 64Kbps is 1.472Mbps.
T1 with SF ZCS will deliver old E&M voice and data in 56Kbps per 24 DS0s. The payload is 1.344 Mbps. ISDN will not work.