As discussed in another thread, alarm systems communicate over phone lines using a rather primitive protocol, consisting of a few audio tones to handshake and say "goodbye" (the alarm protocol calls the latter "kiss-off"). In-between the handshakes, the actual alarm event is transmitted using touch-tone digits (DTMF). This technique has been around for over 25 years, and it wasn't designed with error-correction (beyond retrying a few times before giving up). So, it requires a high-bandwidth codec and relatively clean, clear tones. Unless you've changed it, I believe OBi with Google Talk is configured to use the G.711 codec, which is what you want, and you should also try changing DTMFMethod from the default (Auto) to "InBand", just to ensure that the tones are being played over the voice channel (I don't know how it behaves in "Auto" mode). This is the same way the tones would be sent over a classic POTS/PSTN line. This is not the same DTMF setting as discussed earlier. See my screen shot for details.
Let us know if this helps. If not, as discussed before, I'd suggest trying a quality VoIP provider like Callcentric, configured as your other ISTP, instead of using Google. Again, use the same recommended settings (G.711 with in-band DTMF).
*Disclaimers: 1) I do not work for an alarm company, nor for Callcentric. 2) I have not tried this on my own alarm system -- I keep a PSTN line for this purpose. 3) YMMV, and per OBi's own documented warnings, GV on the OBi device isn't intended for emergency communications (which alarm messages could be considered).