For anyone interested:

U631 - ECM settings
U633 - V.34 settings
U650 - G3 settings

I was able to manually send a fax but couldn't get it to send on its own. I tried all different settings in those modes in various combinations without success.

What did end up fixing it was in U660 under EXCHANGE, I switched it from PSTN to PBX and it worked fine after that.

Thanks everyone for the help!