cisco 7821 phone one way audio issue

[gushuhua]

There  are  two phones which of the type is cisco 7821 and the cucm version is 10.5.   When Phone  named A called the Phone named B, the phone A can not hear the B's  audio, the phone B hear the A's audio.  

I  sniffer the packet of the two phones and found the Phone A can not receive the B's RTP  packet  . The Phone B has sent the RTP packet to the Phone A , but actually the mac address of the phone A  is not correct. How does it happened?

Why the phone B's  arp table  can not update automatically and how can I  resovle it ?

[Adam Pawlowski]

Phone A's MAC should be either the phone itself on the same subnet, or the MAC of a gateway/firewall/etc that's in front of it. A gateway/router shouldn't be selective to the traffic in most cases. A firewall may, if it has an ALG, "helper", fixup, etc, or a ruleset which doesn't permit RTP ingress.

If they're on the same subnet this implies an addressing conflict, transparent firewall, or perhaps they really aren't on the same subnet as intended.

Just a starting point.

[gushuhua]

 These two phones are on the same subnet and not have  an addressing conflict.  There is no  firewall between the two phones.  Phone A  has sent a arp request to the phone B and the phone B has received. After the phone B receviced the Phone A's arp request, it sent an arp replay to the Phone A telling its mac address.  Normally the phone B will update its arp table  when it had received the arp request , but it  did not. The phone B still sent the rtp packet using the old mac address which is another ip phone .

So Why  the phone can not update his arp cache when it received the phone A's arp request?