incoming call no ring sound for caller

[stin]

Use caution with the above solution, this should only be needed in very rare use cases and you could well be masking another problem that may come back and bite you.

I presume the number of possible causes is too large to go into? :/

 

[Alex.K]

Instead of instant_ringback=true try action answer. It works for me. I've faced this issue but only if calls income from specific operator.