I don't know a definitive test to check if the phone port is damaged or not but the following might give some hint.
Log into the OBi's local web server. (You'll need to look at your router's management GUI to see what address the OBi100 has been given.) Password and username are both admin if you haven't changed the latter. With a phone plugged in but on-hook, click on the '+' to expand the 'Status' menu, then click on 'Phone Status'. Note down the values of State, LoopCurrent, VBAT, and TipRingVoltage.
Then take the phone off-hook, click on 'Phone Status' again and note down the values.
Report back here with both sets of values.