The alarm panel is looking for the ~48VDC on-hook voltage on the phone line, and if not present, it'll complain. So, yes, just plugging in an OBi's phone port will satisfy the requirement. However, when you do get an alarm, or when the system does a periodic self-test of its communicator, it'll fail, if that phone line isn't able to actually communicate with the central station. So really, this isn't going to be a good solution.
By the way, the phone line detect feature is optional on most Honeywell/Ademco panels (you can disable it). Off-topic for this forum, but if you could get the password reset, or find out the installer password, you can turn off this feature.