Which VOIP provider are we talking above? GV, Callcentric, Anveo, etc.?
You might want to change your settings. Try changing:
ITSP Profile X>>>>>>General>>>>>DTMFMethod: InBand (X = A or B)
I would setup a Free DID from Callcentric on SP2 and make the change above. There might be some other changes that might make the signal pass through relaibly to your Monitoring Center. Read this post to see if Callcentric can help with getting you the correct settings:
http://www.diyalarmforum.com/board/index.php?topic=16944.0Quote
Note: These Settings might not apply to the OBi, but similar settings might be appropriate:
Callcentric helped me quite a bunch to setup my ATA. Initially the CS received the call but the alarm code would not go through. The deal was that the ATA was not setup to send audio tones. IDK what format they are but they sound very much like dial tone.
The only modifications to the ATA were:
1. Send DTMF : in-audio
2. Fax Mode: pass-through
After this the CS got the messages "loud and clear" as they said.
Do you have a spare ATA (PAP2T-NA, SPA2102, etc.) lying around that you can use instead of the OBi? Maybe you can dedicate this unit to the Alarm System, since some changes to the ATA for reliable Alarm signal might conflict with the voice calls. A dedicated unit would need to seize the line and can be connected directly to the Alarm Panel/Unit.
EDIT: Do you know what alarm reporting format your Simon XT security system is using and what options are available that is VOIP friendly?