[cisco-voip] Call Failure

[Wes Sisk]

If call preservation were enabled for the call then the phone would stay 
offhook until the user initiated a disconnect.  This leads me to 2 thoughts:

1. The call does not have call preservation enabled to begin with.  If 
this is true then as soon as the phone loses CM registration it will 
clear the call.  This is possible depending on the type of call 
(conference, transfer, mtp, internal, external) and the signaling 
protocols involved in the call (h323,cti call preservation disabled by 
default.  h323 can be enabled with service parameter).  Possibly the 
easiest way to verify this is to setup a call between 2 phones in india 
and then flap the WAN connection.  The call should stay up as SCCP 
endpoints always have call preservation enabled.  The most likely 
culprit here is the gateway signaling protocol. If h.323 then change CM 
service parameter/enterprise parameter to enable h.323 call preservation.

2. If call preservation is enabled and the call still automatically 
clears I only know of 1 thing that would cause this - ICMP messages on 
the network.  Gateways and phones both look for ICMP unreachable 
messages and terminate preserved calls if a threshold number of ICMP 
messages are received.  When your WAN flaps does it flap in such a way 
that the endpoints (gateway,phone) would receive ICMP messages?  I do 
not know of any way to change the ICMP behavior on endpoints.  However, 
you could creative in the network and have ICMPs suppressed for RTP 
streams.   This would allow the endpoints to continue to preserve the 
call.  Be careful though, this can also lead to orphaned call legs.

While these may be technically viable what are the chances your external 
callers and users will hang on the line through 10 seconds of no audio?

/Wes

On Thursday, May 28, 2009 9:06:31 AM, Aman Chugh <aman.chugh at gmail.com> 
wrote:
>
> I have a remote location which registers to the CCM at the central 
> site , all the PSTN traffic exits out of the central site gateway and 
> we do not have any PSTN link at this remote site. This site is an 
> extension of my central site. I have situation here as the my data 
> link to this site flaps for 10 seconds resulting in all active calls 
> to drop.
>
> I am using CCM 6 at the central site. Is there any way I can keep the 
> call up when the link flaps. I know the call survives only when IP 
> Phone losses it's link with CCM  but in my case the entire data link 
> goes down. 
>
> I know it is not possible as the entire data link goes down but out of 
> curiosity want to know if this is remotely possible.
>
> TIA,
> Aman
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