UC 540 has one FXO and one SIP trunk. Incoming calls on FXO are pointed to AA extension 500. If you press 4 during AA, it is supposed to use SIP trunk to dial external cell phone. External call transfer is enabled and 9 prefix is configured before cell phone number.
When we test, we hear a busy signal.
If you dial AA extension from internal IP phone and press 4, it does use SIP trunk and ring cell phone succesfully.