The ACK was never received.
Then comes the interesting part: discovering why.
Here are some notes about what should happen, in particular when there are multiple proxies along the path, and with a little additional complexity of one of the proxies with two network interfaces. All this assuming loose routing everywhere. The main reference here of course is RFC 3261.
Isn't an image worth a thousand words? Then here's a sequence diagram:
All Record-Route headers are assumed to carry loose routing URIs (they have the ;lr attribute).
B, C and D, working as proxies that want to stay in the path, record route themselves. For this reason E, the UAS and "callee", receives an INVITE with a list of Record-Route headers with B, C and D.
In particular for B there will be two Record-Route headers, since B is using two separate interfaces, one facing A and the other facing C.
In typical cases the two interfaces represent the interaction with the public Internet on a side and a private infrastructure on the other. But it's not important for this discussion.
Omitting provisional responses for simplicity, let's assume E responds immediately with a 200 OK. This response will have the same list of R-R headers, in the same order, as received by E.
E will also add its URI in the Contact header of the 200 OK.
In this loose routing context the IP address in E's Contact's URI will be relevant only for D in the future.
D, C and B don't modify the list of Record-Route headers, and A receives it as sent by E.
Apart from the operations related to the media session set up, A will send the ACK to the 200 OK.
This ACK will have a Request URI with E's Contact URI (stripped of anything that can't se inside a Request URI), and a Route header list which is basically the received Record-Route header list inverted (see images).
A is saying: "Route this ACK to E, routing it via this list of hops".
When B receives that ACK, it must recognise that the topmost Route headers are B itself, remove them from the Route list, and pick b2 as the interface to deliver the ACK to C.
C and D will have an easier task to remove a single Route header, the one representing them, and deliver the ACK to the next route.
For D, the next route will be in fact E, and the ACK will be routed using only the Request URI, as the Route headers have all been eliminated. This is the only step where the IP address that E has set in the Contact of its 200 OK response needs to be visible by another entity, namely D.
ACK routing as explained in RFC 3665
To further reiterate this concept, let's look at a somewhat simpler example in RFC 3665.
The ACK part is:
You can see there's no requirement for Proxy 1 to be able to reach the UAC contact (client.biloxi.example.com might be completely unreachable from Proxy 1).
It's Proxy 2's responsibility to route the ACK in the last hop towards Bob.
Proxy 1 must leave the R-URI as is (see below for more details on proxy behaviour), strip itself from the list of Routes and route the ACK to the new topmost Route (Proxy 2).
Proxy 2 will strip itself from the Route list, being the topmost Route, and forward the ACK to Bob. There are no more Route headers.
Only at the last hop Bob's contact reachability is relevant, and it is for Proxy 2 only.
More about the behaviour of the proxies to corroborate the ACK routing
From RFC 3261, 16.4:
“ If the first value in the Route header field indicates this proxy,
the proxy MUST remove that value from the request.”
From RFC 3261, 16.5:
“ A proxy can only change the Request-URI of a request during
forwarding if it is responsible for that URI.”
APPENDIX - Why is the ACK to 200 OK to INVITE a separate transaction?
From RFC 3261, ch. 17:
The reason for this separation is rooted in the importance of
delivering all 200 (OK) responses to an INVITE to the UAC. To
deliver them all to the UAC, the UAS alone takes responsibility
for retransmitting them (see Section 126.96.36.199), and the UAC alone
takes responsibility for acknowledging them with ACK (see Section
188.8.131.52). Since this ACK is retransmitted only by the UAC, it is
effectively considered its own transaction.