But there was a limitation: CSeq was not incremented for the INVITE sent with credentials - which was making authentication not-fully RFC compliant. The authentication worked if the other party was using Kamailio (or SER/OpenSER).
The dialog module in Kamailio 4.2 has now the capability of detecting that the authentication was done by Kamailio (via uac_auth() from uac module) and increment the CSeq for the INVITE with credentials header. Subsequent messages in the same dialog will get the CSeq value adjusted accordingly.
This feature has to be enable by setting dialog module parameter 'track_cseq_updates':
modparam("dialog", "track_cseq_updates", 1)
Note that the call (starting with initial INVITE) has to be tracked with dialog module (i.e., use dlg_manage() for requests belonging to that call).