Hi Danny,
Making a big assumption based on the info and diagram.
Looks like you may be trying to integrate the new SBC and migrate from the old PRIs or is this a greenfield install? You mentioned it being a new setup, but I'm assuming CUCM has been in production since it's running on 10.5 and the new component is only the SBC?
Please elaborate on the question above and we can provide additional assistance.
Making big assumptions based on your diagram and information provided
The fast busy signal is most likely coming from your internal Voice Gateway/Router that the SBC connects to. Can you provide the output from your internal VG/Router? show-running config on the VG would be useful. It does look like the outbound call may be going where it needs to, but is it pointed directly at the SIP Trunk or is it a route list?
The output you attached from DNA is showing what looks to be an outbound call, but your original post sounds like you're referring to an inbound call?
Finally, if all the assumptions above are correct the full inbound call flow based on your original post from an external cell phone would look like this:
From cell Caller dials Main Number/DID ---> SBC ---> Internal Voice Gateway/Router ---> SIP Trunk to CUCM ---> In CUCM a Translation pattern, CTI Route Point, etc accepts the call ---> CUC SIP Trunk ---> CUC Forwarded routing rule is matched ---> Then routed to the Auto-Attendant ---> In CUC Forwarded routing rule is matched ---> Routed to the Auto-Attendant
There would need to be digit manipulation among other things to make this work.
The outbound call flow to an external number would look like this:
Cisco IP Phone ---> Route Pattern ---> Route List ---> Route Group ---> SIP Trunk in CUCM ---> Voice Gateway/Router ---> SBC ---> External Called Number.
Again there are too many variables and the majority of the time if the issue is on the provider side you would not receive a fast busy, but a message from ATT, Verizon, etc.
