No Ringback on direct dial IVR from IAX2 Trunk

I have an IVR with both direct dial and single digit selection and am reaching the IVR from an IAX Trunk (G.729). When dialling into the IVR I get the prompt normall but when selecting a destination with a single digit or dialling an extension number there is no ringback tone untill the extension answers or VM and I have a normal conversation.

To test I have set up a DAHDI Trunk to dial into the same IVR and there is Ringback on all calls.

This is specific to IAX2 Trunks.

I am running * with all the latest dahdi and addons.

Pls help because this is a function that we need with an IAX2 Trunk. The very strange thing is that I found that the very first time after a server reboot on the very first call I get ringback even when coming in from the IAX trunk but after that first call I never got ringback again.