[Help] Phone calls get partially dropped

I am having this problem where if for instance I make a call out and during that call I recieve a call that will go to voicemail, my call that I made will get partially dropped where the person I called can still hear me but I cant hear them. As far as I can tell, it might be an issue with Dial never passing the call onto priority n +101 because the call that interupts my existing call ends up going to
exten => 116,3,voicemail(u116) instead of exten =>voicemail(b116). Also, ive tried using ChanIsAvail but it doesnt change anything.

Im using ATCom 320 phones with IAX. Im using Digiums TDM400 card with 2 fxo ports
My Dialplan is:

exten => 116,1,ChanIsAvail(iax2/116)
exten => 116,2,Dial(iax2/116,15)
exten => 116,3,voicemail(u116)
exten => 116,103,voicemail(b116)

Ive not found anything similar anywhere in the forum or mail lists.
Any help would be greatly appreciated