I have a UC520 with 4 FXS Ports each connected to an old fax machine. Incoming faxes works ok, faxing internally to the extenstions also works ok.
Faxing to the outside world over SIP trunk is problematic. I do put the 9 prefix to get outside line and can see the call being established.
It does not complete the transmission and carries on in an loop trying to send it.
On the remote fax, in one particular case was my Hylafax server, and I got this from the log.
MODEM TIMEOUT: waiting for v.21 carrier
I also tried to call my my mobile from the fax machine, and I could.
voice service voip
allow-connections h323 to h323
allow-connections h323 to sip
allow-connections sip to h323
allow-connections sip to sip
no supplementary-service sip moved-temporarily
no supplementary-service sip refer
fax protocol none
no fax-relay sg3-to-g3
registrar server expires max 3600 min 3600
station-id name fax1
station-id number 301
station-id name fax2
station-id number 302
station-id name fax3
station-id number 303
station-id name fax4
station-id number 304