How do I force FAX to use T.38?

Hi,

I managed to get the fax receiving feature started to work so far with G.711. I am wondering how to let the fax to use T.38 instead. Below is the dump of fax statistics, 8 times failure was due to my improper trunk setting, and after I corrected the error, I was able to receive fax 3 times in success, but with using G.711, not T.38.

I would appreciate if anyone can share the insight how to force Fax to use T.38 rather than G.711.

freepbx*CLI> fax show stats

FAX Statistics:

Current Sessions : 0
Reserved Sessions : 0
Transmit Attempts : 0
Receive Attempts : 11
Completed FAXes : 11
Failed FAXes : 8

Spandsp G.711
Success : 3
Switched to T.38 : 0
Call Dropped : 8
No FAX : 0
Negotiation Failed : 0
Train Failure : 0
Retries Exceeded : 0
Protocol Error : 0
TX Protocol Error : 0
RX Protocol Error : 0
File Error : 0
Memory Error : 0
Unknown Error : 0

Spandsp T.38
Success : 0
Call Dropped : 0
No FAX : 0
Negotiation Failed : 0
Train Failure : 0
Retries Exceeded : 0
Protocol Error : 0
TX Protocol Error : 0
RX Protocol Error : 0
File Error : 0
Memory Error : 0
Unknown Error : 0