Cisco isr 4331 with IOS and sip busy fail over to 2nd sip number on pbx?

[CPM86]

I have a fun project going with a few phone systems and a Cisco router.
I have a Cisco ISR 4331 with an IP office 500v2 in partner Edition. This allows me to use three sip channels.

Very simple with no usernames and password. Phone system dials into the Cisco and get routed. Then when I call the phone system based on ip number The call goes through no problem. I have three numbers. 1 per each number.

(Cisco Dial Peer)

dial-peer voice 15 voip (In all I created 3 dial peer for the 3 sip channels.)
destination-pattern 5555555 --- phone number
session protocol sipv2
session target ipv4:0.0.0.0 --- ip number of ip office


Now the problem is. If I call line one. say 555-0001 and put it on hold. When I dial the number again. Instead of failing or (forwarding) over the line 2 says 555-0002. I get a busy signal.
I'm also not sure if this would be on the phone system end via the IP office or the Cisco. I do know on my other IP office running a T1 pri circuit. It is all on the Cisco end and goes to the next open channel. On a side note. Since I refreshing my memory on this. I'll play around with the hunt group on the ip office. See if that makes a different or not.

Any help would be appreciated.

Thank You,

Josh