The settings you may need to change are under Physical Interfaces, LINE Port, PSTN Disconnect Detection.
I don't know the details for UAE, but you can find out with some simple tests. Ideally, do these tests with a line-powered corded phone, connected directly to the house line (OBi temporarily disconnected). For each test, call in from your mobile, answer the corded phone, then see what happens when you end the call on the mobile.
The first question is whether there is a brief interruption in loop current. This is called CPC. Press and hold a number button on the corded phone, then hang up the mobile. If the tone stops playing for a fraction of a second, you have CPC and setting CPCTimeThreshold to a lower value (try 250 or 150 milliseconds) should help.
If there is no CPC, what do you hear (not pressing any buttons) after ending the call on the mobile? If there is a beeping sound, that's called Disconnect Tone. Measure the frequency(ies) and cadence, e.g. by recording it with your computer mic and examining it with an audio editor. In my case (France) the disconnect tone was 440 Hz and played for 0.5 seconds, once a second., so I set DisconnectTonePattern to 440-30;10;(.5+.5) and it fixed the problem.
If there is no disconnect tone, do you hear dial tone? If so, you could set DisconnectTonePattern to recognize a few seconds of dial tone.
If none of the above are available (you just get silence), you could reduce SilenceTimeThreshold to make it disconnect faster. But be careful; if you set e.g. 30 (seconds), then if you are silent for 30 seconds while your contact looks something up on the computer or goes to get something, then the OBi will disconnect you.
