[Serusers] SER Proxy auth

Hello, I'm currently trying to setup a SIP environment for VoIP calling for my final school project, so I'm just working with VoIP/SIP for 2 weeks. I'm using SER as a SIP proxy server, but the carrier/gateway I am using for calling to/from PSTN is requiring me to register at their server and authorize outgoing calls, which is something SER won't do. So I got the idea to use asterisk between the PSTN carrier and SER for the authorization, since Asterisk can register and auth itself. SIP --------- SIP --------- SIP --------- PSTN -----| |-----------| |-----------| |------ --------- --------- --------- SER Asterisk Carrier <-- auth stuff --> <-- sip relay --> Has anyone here ever tried a similar setup or is this an impossibility ? Kind regards, E. Versaevel