I am guessing that since you reference the TAMB that your paging system is a Bogen. When I have researched FXS integration with Bogen in the past, a TAMB was required. I am hedging a little here because it may be platform specific (i.e. Bogen platform dependancy). You would need to check specific part information. I have also seen the symptoms you describe. I don't recall if it was a Bogen integration or not. The eventual solution in that specific scenario was to use FXO on the Cisco-side.
I'd recommend that you identify the head end model number for your paging solution. Check to see if it requires anything when integrated with a device that is providing voltage on the line (as this is what an FXS port does). That should get you closer to your answer. Of course, you can post and or search for your paging solution model number in the forums. You will find plenty of threads on the topic.