I to would like to know this.
My belief is that SIP and fax don't go.
We have "rightfax" using SIP trunks with T38 but this is ISDN to 3300 then SIP to Rightfax server
That works fine as long as you buy the T38 licence and the DCP II card.
Probably cheapest to put alog line in. You would probably find they have one already for the ADSL or Red care Alarm line.
Sorry i could not help, good luck