we have setup Flowroute SIP trunks that traverse through our Firewall. the calls setup and tear down just fine but there is no secondary dial tone or ring back. i have it setup for test purposes to use a 9 to route the SIP trunk. ARS Digits Dialed states the following
9 followed by 7 digits route to SIP trunk
91NXX followed by 7 digits route to SIP trunk
911 followed by 0 digits route to PRI
our default setup has 8 as the access to the PRIs
dial an 8 then we get secondary dialtone, then dial the remaining digits and it rings.