Juha,
There is the parameter restart_fr_on_each_reply, its documentation says:
Setting it to 0 is especially useful when dealing with bad UAs that continuously retransmit 180s, not allowing the transaction to timeout (and thus making impossible the implementation of certain services, like automatic voicemail after x seconds).
The parameter is enabled by default. Shouldn't we disable this when we do serial forking with t_next_contacts. To guarantee that UAs retransmitting 180s do not prevent serial forking from happening?
-- Jan