Virtually all decent sized comapnies use ISDN PRIs for their bigger voice trunks instead of T1s.
Actually the current trend is to drop the PRIs and migrate to DS1's into channel banks be they an external CB or part of the subs switch or to VoIP over DS1s DS3s or Metro-Ethernet.
Heck simply going from a PRI to a T gains one 64K as the D channel is no longer needed and that bandwidth can now be rolled into the overall pipe.
Wayne -- If you cannot fix it with a buttset and some beanies you ain't a technician.