Fax Transmission is sensitive to delay, and you can use some mechanisms to transmit fax over WAN links.

cisco fax relay

T.38 fax relay

fax passtrougth (using G.711 Codec) at 80 Kbps aprox.

or the fax rate configured in dial-peer in a specific bandwith or in voice rate, maybe you can manage VAD Active or Desactivate in some cases.

im using onramp and my concern is to restrict simultanius incomming faxes depending on the resource consumtion by this service.