HI everyone !!!

I have 2 PRI one is form telco(Phone Company) an other is virtual through SIP Trunk, for calling card, the main dialing number is on PRI form TELCO, i would like to forward the incoming call to virtual PRI if first PRI from TELCO shows busy all channels , i am using A2bliling as final destination for calls, i have tried through Extention follow me and fail over option , tried though Queue , but could not find any hope, is there any who can help or any idea where should i try to get this.

if i could not explain ,i will more , please if any one has idea please share

thanks and regards