[Asterisk-Users] Anyone knows how to receive a SIP call without registering gateway?

[C. Savinovich]

   Hello everyone, I am pulling my hair here because a carrier threw me curve early today.

   They want to send calls to my asterisk server using SIP.  Then they said that their gateways don't have to register with my server, that all they have to do is send a prefix for validation.  Whereas I can think of several ways to authenticate their incoming number string, I am only used to the orthodox SIP way which is: client registers to my proxy.   Guess what, I can't find any samples on this!!, Can anyone please help?, I will probably need a sample sip.conf.   and then, to make a test call, I can use another asterisk box and try asterisk to asterisk sip calls (without register) via the cli prompt.   But I have no idea.... and I am intrigued.

   Thanks
   CS