I now know that if you're using a line enabled OBi, you can use:
li('') to route all incoming calls to the PSTN dial tone.
Is there, by chance, a way to route inbound calls to SPx dial tone, without some sort of AA trickery?
No, you can't get dial tone from an SIP provider.
You can modify the AA to only play the "Welcome" message, so it's not as annoying.
Voice Service -> AA Service
MenuTitle: &pause()
Menu: &pause()
PleaseWait: &pause()
EnterNumber: &pause()
After getting the Welcome message, dial 2 number #
You can record a dial tone and replace the Welcome message with this recording.
You can't change the 2 before or the # after the outbound number.