Can I use the TE420 card on a T2 line ?
I dont think so. I dont know of any digital cards available for more than T1’s. My guess is companies that need tha kind of density would use gateways for load balancing and redundancy anyways so that is probably your best place to look. For gateways I have heard very good things about AudioCodes devices, they come highly recommended by several consultants I have talked with taht implement large Asterisk systems.