@samp010
I have always told people that lack of dial tone is usually a hardware failure. The phone cord, the phone or the OBi. It is possible to customise the tones including dial tone so there is the remote possibility that it has somehow been configured with parameters that yield silence. That is why I also tell people to look at the voltages under Status, Phone Status. I believe there are no user configuration options which can set those to zero or to return blanks on that page. I also doubt that there would be any (hidden from the user) software controls which could affect those voltages but I have no evidence to prove that so need to keep an open mind about it.
However the recent posts by SteveInUS (not to be confused with someone else with a similar username) say that there are circumstances where lack of dial tone is not caused by a hardware failure and that dial tone is restored by a reset to defaults. (Note that SteveInUS's posts have not said anything about the values reported on the Phone Port Status page.)
You have nothing to lose by trying a reset. You will need to use the button on the underside of the OBi as detailed at
https://www.obitalk.com/info/faq/Troubleshooting-sec/How-to-Factory-ResetI admit to being doubtful that this will restore dial tone and the missing voltages in this case. OBis contain a DC to DC converter to obtain from the 12 volt wall-wart the 50 volts for the phone interface. It is a fact that the time when failing DC-DC converters (not just the ones in OBis) finally stop working completely is most likely to be when input power is reapplied. Did your OBi have its power interrupted either deliberately or inadvertently when the modem was changed?