There's effectively five things that can happen when you attempt to go for an external authorization to a transaction endpoint:
2. You can find that none of the channels are connected.
3. Your request can time out (partner didn't respond in time).
[APPLERR_QUERYHOSTTO – note that depending upon the interface and your mutual agreement with the partner, you may be obligated to send a conditional reversal of the original request via a SAF queue. See an earlier write-up on SAF here.]
4. The partner can answer you but deny the transaction request.
[Varies by implementation, but in ISO8583, this value is plucked from Field 39 and gets translated into a non-0000 Internal Result Code. See piece here on result code translation]
5. The partner can answer you with an approval.