You are not required to assign a phone number to all of your users with the following implications:
For users with Basic License only, they can receive transferred call from the receptionist but they will not be able to call out(only internally with extension to extension dialing). For users with a calling plans, they can receive transferred call from Receptionist but to call out, they need to use a DID as their outbound caller ID(i.e Main company number set under Phone system Management>Phones>assigned, Site Auto Reception DID, Call Queue DID- as long as user is a member of the call queue)