Had an interesting fault come across my desk the other day; calls were failing when the called party (an SSP we talk to via SS7/ISUP) had an exchange based call forward in place.

In SIP, we can do call forwarding one of two ways, we can send a 302 Redirect or we can generate a new SIP invite.

But in ISUP how is it done?

We’re a SIP based network, but we do talk some SS7/ISUP on the edges, and it was important that we handled this correctly.

I could see in the Address Complete Message (ACM) sent back to our network that there was redirection information here:

We would see the B party SSP release the call as soon as it sent this.

This made me wonder if we, as the originating network, were supposed to redirect to the new B party and send a new Initial Address Message?

After a lot of digging in the ITU Q.7xx docs (I’m not where near as fast at finding information in specs written prior to my birth, than I am with the 3GPP specs) I found my answer – These headers are informational only, the B party SSP is meant to re-target the message, and send us an Alerting or Answer message when it’s done so.