Are you doing Stateful or Transparent Proxy in your TA900e?
If so then this document goes over how to do this assuming you are using the FXO fulltime:
You must have these global commands entered:
Voice forward-mode local
sip proxy failover direct-inbound
You must have the ‘prefer trunk-routing’ command on the analog trunk account.
The TA 900e will try to route the call out the sip trunk to the softswitch, if it fails, it will timeout, then get routed to the switchboard and the local ring group will take over and route the calls to the proxy users.
Let me know if you have any questions.