Cisco VOIP Phones dropping External calls

I have a Cisco phone system that was recently installed.  Everything seems to be working fine, but we are having issues with the system randomly dropping external calls.  The call still shows as active but no audio can be heard.  I've got a Gateway router 2821 model that handles all of the call routing, and Cisco Call manager  VOIP to VOIP internal calls seem to be fine, and outgoing external calls seem to  not have a problem, but incoming external calls are pretty hit an miss.   Any suggestions?
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it seems a firewall issue...
breckdahlinAuthor Commented:
I thought it might be that as well, but the voice PRI is connected driectly to the router.  Our pix is not an issue with voice data
can you check if inbound leg is reachcing your firewall and how it treats it?
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breckdahlinAuthor Commented:
The calls come directly into the router from the CO.  There are 3 PRI's that are directly connected to the voice gateway router (2821) and from there are sent to Call manager.  the firewall is not in play in this instance
Is your gateway h.323 or mgcp?
What codec are you using between call manager and the router?
breckdahlinAuthor Commented:
The gateway is H.323

the codec is
codec preference 1 g711ulaw
 codec preference 2 g729r8
What version Call Manager? Try resetting the h323 gateway in CM?
breckdahlinAuthor Commented:
Cisco Call manager

I will reset the gateway after close of business today, and let you know
Hi there

this is a Codec issue. are the phones in the same location?

breckdahlinAuthor Commented:
yes all of the phones are in the same location.

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