Jawad,
Is there any possibility that SER requests radius server to check if a user should be allowed to call a specific user.
Check if a user should be allowed to call - Yes Check if a user should be allowed to call a specific user - No
I mean as SER requsts radius upon REGISTRATION, can it send similar radius request upon INVITE?
Yes, just use proxy_challenge() again. It's the same logic for REGISTER.
Zeus