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SIP retry on 503/isdn 63 release code

I have the following setup:

(caller) ISDN PBX ------ cisco 5400 ios 12.4(1c) ------ SIP SBC (callee)

if a call is being made originating from the PBX, towards the SBC end, and for some reason the SBC replies to the 5400 with "503 unspecified/service unavailable", the 5400 instead of dropping the call keeps retrying by sending new SIP invites to the SBC, until the caller hangs up. How do i prevent this? i.e. how do i force the 5400 to send a user busy/fast-busy response to the caller (or terminate the call anyway) after 2 attempts?

i can't find a command regarding this except for "no voice hunt" (which doesn't seem to do the trick)

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Re: SIP retry on 503/isdn 63 release code

I think the router is right in retrying because 503 indicates a temporary conditions.

However, it is possble to customize sip / isnd response mappings, not sure if that would help.