Connect it to an analog port on the MiVO 250 if at all possible...
Remember this system does not support T.38. To make faxing work using G.711 you need to have things setup near perfectly with your trunking. If it's SIP trunks, your likely not going to get it working. If it is (real) analog trunks, you need to have them properly setup for levels and impedance/echo cancellation and it might work. If you have a true digital PRI (not a SIP PRI) you might actually get this to work, but without doing anything special, it should just work or not work.
In my experience, faxing through the 5000 with anything but a traditional PRI has been dodgy at best, but with the proliferation of SIP based services, faxing is becoming a more and more difficult thing to deal with especially on this system.