ringback tone

Hello everyone, for some reason I have no ringback tone when someone calls into a DID. As far as I can see it could probably be a early media problem? Any advice would be appreciated. Thanks.

This is sort of crazy, maybe someone has faced the same. Some (but not all) extensions in a certain domain stopped sending ringback tone when dialed to. It was consistent for these extensions and reproducible in a simple extension-to-extension call. I could see valid early media SDP in both...