Ringback tone is delayed when calling to PSTN through a trunk

When I place a call through my trunk to PSTN, the phone being called rings several times, but the phone placing call don’t get any ringback tone until approximately 20 seconds! why?

So I don’t know what is going on until somesone on the other end picks up the phone, or until after 20 seconds !

It’s very unconvenient!

Can anybody help?