This paper studies the economic effects of telecommunications policies within a vertically differentiated duopoly. VoIP(Voice over Internet Protocol) has been newly introduced in telecommunication market where the incumbent firms have provided PSTN(Public Switched Telephone Network) telephony service. The government adopted the minimum quality standard for VoIP operations to increase the voice qualities serviced and consumed. Theoretical explanations for basic minimum required voice quality standard policy have been developed in the game context of PSTN telephony which represent the incumbent high quality service and VoIP which represent relatively low quality service. In this vertical product differentiation, where consumers derive a benefit from quality, PSTN and VoIP firms determine the competitive prices. In equilibrium, the government regulation for voice quality lowers the PSTN price. The price of VoIP, however, rises only when the marginal cost is relatively high. Moreover, the minimum required quality standard enhances both PSTN and VoIP market but lowers the profit of both providers. Thus regulatory formulas which mandate a minimum quality have been found to be welfare improving. I also extend the basic model by adding the regulatory scheme of VoIP-PSTN number portability. The government recently planned to begin the number portability system which enable users to keep current local phone number of PSTN telephony for VoIP based calls. This policy is designed to encourage the VoIP market since the current 070 VoIP number caused to hesitate VoIP subscription. The analysis demonstrates that the number portability lowers the PSTN price and raises the VoIP price. Moreover, the number portability increases the demand of VoIP service but shrinks the PSTN service. As a result, the PSTN telephony’s surplus is transferred to the VoIP service provider through the minimum required quality standard and number portability policies.