56K modems were never meant to direct connect to each other when both ends were terminated on analog POTS lines. I remember having to explain this to people back in the day that wanted to connect house to house or connect branch offices faster than 33.6K. Service providers had dialup Points of Presence that had incoming digital connections and could negotiate 56K. The upstream connection was established at 53K to 56K (under the right conditions) and the downstream connection back to the user was 33.6K max (analog.) David was right and you will definitely need to simulate a digital connection to get 56K.
At the last VCF East I think the people with the display "Serial Ports" might of had a digital modem bank. Livingston Portmaster 3, Lucend Ascend Max or something from Cisco probably. USR for that proprietary x2 stuff. Of course you will need something to go from a T1 or PRI over to POTS. Not sure why two 56k's back to back couldn't get there though, it's not like there is a long line loop between them.