Assuming the system is programmed correctly as Ralph indicated, this is controlled by the Protocol variant (Bellcore, DMS100, 5ESS, etc). You would need to match the 3300 to what the carrier is providing, most current PRI circuits in the USA use Bellcore, but in the past it could be almost anything so if the PRI was existing on the old system you will need to find out the protocol variant used and match it in the 3300, or have the carrier change it to one the Mitel supports.