Brother MFC - L2750dw - Fax problem

[Underwood]

When I connect the telephone cord from the female jack on my MFC, and plug it into the wall jack for a fax line & fax, it does not ring the telephone
at the destination & puts my router on the same line offline, until I unplug that connection and re boot the router.

Only this feature is having problems. Another MFC - 8820D is on the same fax line and does not have a problem faxing on the line my router is on.
I needed an extension telephone cord. Does it have to be DSL modem cable, two pins & two wire?

 

[Underwood]

Two wire fax telephone wire only as specified by Brother training video. I tried their fix and put in a two pin two wire into their
4 wire jack on MFC. They gave me the idea that fixed my faxing SEND & RECEIVE problem. I used, HIGH compatability instead of their basic, VoIP. Voice of IP is not what we are doing, we are going over the telephone lines.

It is FIXED.