ISP’s do not typically lease out T1’s. This is a function of a local telecom provider. The ISP may have multiple customers coming into their data center where the aggregate inbound customer bandwidth exceeds the capacity of the ISP. For example, 40 different customers have T1’s (total 60Mbps) provided by various telecom organizations yet their ISP may have just an OC3 (45Mbps). This is called over-subscription. An example can be seen here.