There are two types of this phone. The ones with the wireless headset attached to the side are not able to connect with a Busy Beacon (see photo below).
If your Customer has a Mitel 5330IP with an attached headset then they cannot use a Busy Beacon(DISCONTINUED) . Sorry!
Any other model of the Mitel 5330IP should be just fine!
This is also true of the BUSY BUDDY light.