henningw left a comment (kamailio/kamailio#4250)
@henningw no, as @miconda mentions.
The ACKs for 200ok are end-to-end, should not be generated by Kamailio, just forwarded (stateless). But the ACKs for 300+ replies of received+relayed INVITEs are hop-by-hop, and they should not trigger by default the event route (it can be via a modparam).
In summary, UAC is only responsible for generating ACK for 2xx responses. ACKs to Error responses are in-transaction (and not in dialog) and should be a completely different feature if desired.
If the Kamailio generates the INVITE, the non-200 OK ACKS are originated from the Kamailio. But I agree, they are not a separate transaction. I just checked the tm module documentation, it is also documented like this, great. Xenofon is reviewing the CANCEL case, but that should be probably discussed elsewhere.