Innocent Evil wrote:
So the key point is, both end need to have same codec supported. It doesn't matter how many system/server/broker are between those two end, right?
Well, all systems between, which decode the RTP stream on one call leg and encode it on the other one (like session border controllers, PSTN gateways etc.), have to support the codec of their peer on the corresponding call leg.
If they are pure SIP proxies, they don't get involved into the codec negotiation.
Andy