What is the output voltage of the jack that T7208 phone is plugged into?
Can it damage a fax device if you accidentally plug it into a jack that a T7208 phone is using?

Not sure of the voltage, but its relatively unimportant.

You ought not damage a fax or the system, if you accidentally connect one to another. Try turning the system off for 1 minute, then turning on again. If that doesn't fix it, you need to seek a professional who knows the system.

24vdc

What is your problem?

The only damage that may occur would be the opposite, plugging in a set to an analog line then when the line rings it will be 90 volts being pushed into the digital set.