It's kind of a pain.
I've had fax relay work quite well. in several situations. For fax relay there are several options that affect the success of faxes. This is fax relay using h.323, I don't know how mgcp or sip changes things.
Codec: g.711 ensures the most success. It also works on G.729
NSF: setting this to 00000 forces the machine to negotiate with the gateways parameters. There are other, vendor specific (for the fax machine) settings you could put here.
ECM: if you disable error correction mode you might have better success, as it stops retransmissions and hence the time it takes to send the fax... some faxes time out.
fax rate: setting this to 9600 might help.
VAD: turning off VAD helps
Here is an example with of a dial peer with the relevant commands. I don't know if the vg248 and the 6608 have similar commands. This is on a vg200 running h.323. See the docs at the bottom for information on mgcp and sip.
dial-peer voice 2 voip
codec g711ulaw
fax-relay ecm disable
fax rate 9600
fax nsf 000000
ip precedence 5
no vad
I think everything there except the codec must be set on both dial peers, and you may need to also set voice port properties like VAD or codec.
There some other variables like dropping, etc that might affect faxing, but I don't know about those.
If QoS is bad, any delay or jitter might screw up the faxes just enough to make them fail. If the calls are only travelling over a LAN you could try giving special priority to calls from the voice ports in question.
Finally, you can also use fax relay with T.38. This is supposedly even more reliable, but requires more bandwidth. I think the T.38 command can be enabled like this on a dial peer: protocol t38. You can also define it globally in the voice services section of ios.
Here are some documents:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_access/fxmdmnt.htm
http://www.cisco.com/warp/public/788/voip/fax-relay.html
