Hi: how old is "older"? The original WE 2500 phones had an electromagnetic bell ringer, dating back (more or less the same design) to the earliest rotary phone ringers. The later 2500s and their clones had electronic ringers. The bell ringer may require more voltage to ring than the OBi's default setting of 70VAC.
You can bump it up to 82VAC by going into expert configuration mode, either via the OBiTALK portal or the device's local, embedded web server interface. You need to remove the check marks from both boxes to the right of the setting, before it will allow you to change it. If increasing the voltage doesn't work, you can also try changing the waveform to trapezoidal, but I doubt that's the issue.
I haven't tried ringing a 2500 phone in a long time, but I've got a 1947 WE 302 phone, and it will ring using the default 70VAC sinusoidal signal.
If you can't get your setup to work, and this is the only reason you bought the OBi, and you don't care about the OBi's many advanced features, you can buy a Grandstream HT-701 ATA, which I've tested to work on bell ringers. The latest firmware for that ATA also has adjustable ring voltage.
