Faxing over any digital phone line can be hit and miss. Fax machines were originally designed to work with traditional analog phone lines and don’t always work well over today's digital phone lines.
Making some adjustments to your fax machine settings may help. Some people have managed to make it work by lowering the fax machines baud rate to 9600 or lower.
Another option is to switch to a digital fax service. There are several digital fax services out there, so do your research and find the service that works best for you.