Faxing over VoIP
If it can be avoided Faxing over VoIP should not be used - it is yet to catch up with the technology available for voice calls. Faxing uses a different method of transmission which does not fit well over a voice VoIP connection and even though you may manage to send a single page fax, sending anything larger than this would probably take several attempts.
The reason is when the fax tone is transformed into digital packets, some packets are lost. Packet loss on a voice does not make a great deal of difference as you can still hear and understand what the person is saying. But, when there is packet loss on a fax, any little bit can really affect the fax which is why several attempts may be required to get it right.
VoIP provides to businesses often provide a separate fax line free of charge to allow their users to fax using a different transmission code and therefore make it more reliable.
It will however only be a matter of time before VoIP developers are able to come up with a more reliable faxing VoIP service.