Proxies are absolutely not to alter the To and From header. If A sends a request to B through proxy P, and P alters its To or From headers, those get copied into responses and in-dialog requests from B back to A. A is well within its rights to say, "That's not my To header!" Kamailio nevertheless has that capability: https://kamailio.org/docs/modules/5.1.x/modules/uac.html#uac.f.uac_replace_… It is done in a "clever" way that allows it to conceal the fact that it has been altered from A by restoring it to the original value in any messages destined for A. You can learn more about that in the module documentation, e.g. https://kamailio.org/docs/modules/5.1.x/modules/uac.html#uac.p.restore_mode -- Alex On Thu, Jun 21, 2018 at 10:57:54PM +0300, Igor Olhovskiy wrote: -- Alex Balashov | Principal | Evariste Systems LLC Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free) Web: http://www.evaristesys.com/, http://www.csrpswitch.com/