ISDN Zap Circuit-busy (cause code 42) ONLY when dialing 9

I can successfully DIAL (bridge) to a 4 digit extension.

However, I receive the error ‘circuit-busy’ or error code 42 when I attempt to DIAL a 9 (to get outside line) followed by a 7 digit number.

Using a TE410P connected to an ISDN PRI circuit.

Any suggestions welcome.