This is a strange situation. Make sure none of the ports are in TEST mode. I suggest pulling and re-seating the FXS modules.
If you hear dial tone, then that is coming from the switch, as the TA 750 doesn't provide dial tone. It is simply a channel bank. When dialing, the phone is producing the DTMF tones, and the TA 750 is simply passing it along the channel (DS0) on the T1. Are you able to get a T-Bert connected to the T1 and verify you can dial out using that?
You can go into the TEST mode of the FXS module and generate a 1 KHz tone in "both directions" and see if that is detected at the switch, as well as on a phone or butt-set off the FXS port.
(When you disable the test, you should re-seat the FXS module again to ensure the test gets canceled.)
Hope this helps,