Well, seeing that you have an SX-50, I have to assume that the fact that they are a VoIP network really doesn't matter. They have to be converted to either LS or GS to interface to your system. That said, take a test set or even a single line phone and see if you can make calls that way. If they're GS, you'll need to manually ground the ring lead for a brief second to draw dial tone. Then try the same call that fails over the phone system and see what you get.