The present invention is a method for realizing alternation of dual services in tandem office, the method is applied to the tandem office which exchanges by using internet protocol IP /asynchronous transfer mode ATM. The present method includes: tandem office media gateway reports media type change message to tandem mobile switching center server when it detects change of media type of band information packet of the current call, wherein the media type change message carries the changed media type information; the tandem mobile switching center server judges whether a bearer type needs to be modified or not based on the reported media type; if so, it transmits modify indication of the bearer type to the tandem office media gateway; the tandem media gateway modifies itself bearer type when it receives the modify indication of the bearer type and transmits the changed band information packet. The method can reasonably utilize dynamic band of tandem office and ensure normal execution of the changed service when there is alternation of dual services.
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