I think there is a setting within the Service parameters to force it try the next available trunk. Its turned off by default. We had this problem with h323 gateways in a route group. I cant recall the parameter, but i can search around for it.
Also, make sure you are running the latest and greatest IOS for the gateways... there are many SIP fixes in there. But I think your issue is the service parameter setting.
I think this parameter: (taken from another post)
If your PRI is down then the router should report back to CCM a cause code 1 unallocated number. **Editing because I had the syntax wrong** By default, Stop Routing on unallocated number is set to True, which is the CCM equivalent of that cause code. You need to set this to false since the default stops routing. Assuming your CCM Route lists and groups are working (which I do recommend a reset or the new changes won't take effect) then it should failover to the alternate H323 gateway.
Please configure the following command in the global mode on all your vg's:
"no dial-peer outbound status-check pots"
This disables status checking of an outbound POTS dial-peer.
This way, the dial-peer is matched, even when the ISDN is down (and voice-port), and reports a cause code with a "No circuit/channel available" back to CUCM which then hunts for the next member (vg 3 in your case).
Without this setting, the dial-peer is down and therfore not matched, this results in a cause code with "Unallocated/unassigned number" sent back to the CUCM which stops hunting (giving you a busy).
We had a similar problem with H.323 and CallManager 7.x. Maybe this will give you a clue as to how to fix with SIP
Calls were not failing over to second route group when only PRI was down on only (H.323) voice gateway in first route group. No other H.323 gateways or MGCP interfaces in first route group. Voice gateway is 12.4(24)T.
Cisco TAC had us do the following which fixed it:
For the voice gateway, input the following commands:
voice service voip -> h323 -> no h225 timeout keepalive
On CallManager, change following Service Parameters:
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