I haven’t' seen any thing about T38 however if you have two IPOs connect over IP you can us the Proprietary fax transport upto 14.4 fax speeds. Make sure its version 1.4.

Fax Transport is specified in the VoIP trunk field

If you are using Fax Over IP:-
 
The IP Office has a proprietary method for carrying Fax traffic on a VoIP call, it is supported between IP Office systems and supports Fax speeds upto 14.4 kbps, it does not confirm to the T.38 standard. The bandwidth requirements for the call will initially be as per the specified or negotiated compression method then the bandwidth requirement will change to accommodate the Fax data. The Fax bandwidth will vary depending on the speed that the Fax devices are communicating at and the type of link, at 14.4 kbps the bandwidth requirement will be approximately 27 kbps on the LAN or 19 kbps on a Point to Point WAN link with Header Compression.

Thanks you all for warm help, a project here need IPO connect CISCO, and customer wants Fax over these H.323 trunks. But...
So I don't Avaya will support ITU fax standard in future.