The alert info is part of the SIP header when a call is sent to an endpoint. Once you send the call over the IAX trunk all the SIP headers are lost unless you use custom dialplan to preserve them. I have done this in the past using macro-dialout-trunk-predial-hook and a corresponding context at the far end to decode and reset the header(s). Not exactly simple, but if you have budget for this we can assist in paid support. See this post for similar concept using Caller ID: