At risk of stepping in it again, might I speculate that we actually have the hardware to handle this blocking, but probably not the software/firmware? With the GV forwarding as described, we disable forwarding to Google Chat because we're forwarding to the CallCentric DID number on the same device. But, if we enabled this Chat forwarding then wouldn't we be receiving the same call on two SPs? And if this condition could be detected then that would differentiate from the case where the call comes in only on the CallCentric SP. The reality of having slightly different timing on the two SP calls would make things a little messy, but an exercise in speculation isn't necessarily meant to be practical.