The Wi-Manx service wasn’t designed to support faxes. In some cases it is possible to send faxes over the VoIP system although this practice isn’t supported.
To send a fax you would need to lower the transmission rate on your fax machines and make sure the ATA is configured to use G.711u or G.711a.
“Lossy” codecs such as G.729a will not function correctly.
In the future we will be adding T.38 support which will support the sending of faxes.