I have serious doubts that this will work unless it is a very low baud rate, 1200/2400 bps. I have a feeling that the default speed is higher than that.
Try to se forced only codec G711 ( on ATA ) , delete other possible codec .
POS work at low speed , normally aren't other problem .
If you have Trunk with codec g729 and Ata with codec g729 it possible that work using it , and on g729 modem/fax not work .