Hi, On 02/08/2016 02:49 PM, Andreas Granig wrote: > Any ideas whether it's possible to get the transaction using > pjsip_tx_data and checking retransmit_count there (is this the right > approach in general anyways?) or if patching pjsip to hook up a custom > callback is more feasible? After checking the transaction implementation briefly and experimenting a bit, it seems that at least for REGISTER something like this could be used in the on_tx_msg() callbacks to check for retransmissions: pjsip_module *tsx_mod = pjsip_tsx_layer_instance(); if(tsx_mod && !tdata->mod_data[tsx_mod->id]) { /* looks like a retransmission due to missing tsx, since in normal requests, this always contains the transaction struct!? */ } Will test further with INVITES and others whether this holds true in all cases. Andreas