i don't believe so, the SIP registrar part changed on 6.0 so it was a real SIP proxy, and for the polycom to work properly, it needed the 6.0 on 5.5 you will most likely get redirect errors since it was not a true sip proxy on 5.5 and below. the call presentation and setup is handled by the NRS as of 6, so 5.5 just redirected the IP request to the endpoint which caused the redirect errors on sniffer traces.... the only way it worked from the little time i tried on 5.5 is when you typed the whole URL of the endpoint, you could make a call (but you had to use the SIP dial strings eg. 55664@nowhere.com, other wise it would get fast busy only....) i did not try 5.5 too much, so there might be a workaround, but i did not bother too much as 6.0 worked as per book (but still to a while to setup)
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