A method, system and apparatus for on-demand phone number updating. A system for on-demand phone number updating can include a smooth number transition (SNT) call routing table mapping prior telephone numbers to new telephone numbers for corresponding SNT subscribers. The system further can include an SNT processor linked to the SNT routing table and communicatively coupled to switch logic in a PSTN. Finally, the system can include at least one SNT update table indicating whether different callers associated with a particular SNT subscriber have received an update for a new telephone number for the particular SNT subscriber. For instance, the SNT update table can be coupled to the server-side SNT processor, for instance as part of the SNT routing table, or the SNT update table can be disposed in a telephone system for the particular SNT subscriber.