I'm running into this situation:  Multiple "PSTN gateway boxes".  If a
call is to be routed to a PSTN line, how do I send it to a gateway in
an intelligent way?  By that, I mean:

- If the call is sent to gateway 1, and it has no free ports, and
  (most likely) rejects the INVITE with a 503, the call will be sent
  to gateway 2, etc.

- Similarly if gateway 1 is unreachable, which presumably appears as a
  408.

- But if the call receives an error response which indicates an error
  condition at the far end of the PSTN (e.g., 486), do not retry the
  call on the next gateway.

- Be organized as a SIP forking operation of some sort so that it can
  be integrated as one fork of an ordinary call routing situation.

Ordinary SIP serial forking does not do this correctly, nor does
making the IP addresses of the gateways alternative A records for a
DNS name.  I'm thinking that the only workable solution is to treat
expanding "[EMAIL PROTECTED]" into "[EMAIL PROTECTED]", "[EMAIL PROTECTED]", and
"[EMAIL PROTECTED]" as a proxy operation with a particularly complex
algorithm for building the target set.

Does anyone have a better solution?

Dale
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