A method, an apparatus and a system for allocating a phone number are disclosed. the method includes: receiving, by an SIP server, an automatic number negotiation request packet sent by an IP phone terminal; allocating, by the SIP server, a corresponding IP phone number to the IP phone terminal according to the received automatic number negotiation request packet; and returning, by the SIP server, the allocated IP phone number to the IP phone terminal, so that the IP phone terminal performs a login by using the IP phone number. The method, apparatus and system can improve efficiency and operability of deploying IP phone numbers for a large quantity of IP phone terminals.