Method and system for transferring internet protocol private branch exchange (IPPBX) internal number to external internet telephony service provider (ITSP) public numbers

The invention provides a method for transferring an internet protocol private branch exchange (IPPBX) internal number to external internet telephony service provider (ITSP) public numbers, and a system for realizing the method. The method comprises the following steps that: the IPPBX internal number successfully establishes calling with two ITSP public numbers through an IPPBX respectively, and the IPPBX records the calling information of the two routes of calls; and a corresponding telephone of the IPPBX internal number as a transferring part initiates calling transferring, the IPPBX finishes calling transferring according to the calling information of the two recorded routes of calls.