Inmarsat and 'not that we know for sure', but that wasn't all that important because it did not matter what the contents of the call were (there was a Russian official on the other side of the call so they already knew everything that was said and could ID him) but where exactly it came from and for that the Russians had built a special purpose tracker that allowed them to pinpoint the location precisely enough that a jet sent quickly could make it to the location of the call and fire a missile. Since the Russians knew exactly when the call started they could presumably pick it out of the background and I highly doubt there were many such phones operating in the area to begin with.

His big mistake was to be on the phone for just a little bit longer than it took for the plane to get there, on that day he spoke on the phone for 3 times as long as normal, 15 minutes as opposed to normally 5 or less.