Basically, Nigerian consumers of mobile telecommunications services had to lose their telephone numbers and adopt new ones when switching from one mobile network provider to another and to prevent this ugly situation SIM porting was launched on April 22, 2013 by the Nigerian Telecommunications Commission (NCC). Mobile Number Portability (MNP) is a service that enables mobile telephone users to switch from one mobile network operator to another, in search of better service quality, while retaining their original telephone numbers. The research x-rays the Effect of Subscriber Identity Module (SIM) Porting in Marketing Strategies using MTN SIM Porting as a case study. Critical reviews to related literature were made. Relevant theories were used to gratify the study. Quantitative research design was used in which cross-sectional survey method was adopted while the data collection instrument was questionnaire. Questionnaires were administered to 120 respondents while 110 copies of questionnaires were returned for analysis. Frequency and percentage method of data analysis was used. Mobile Number Portability in Nigeria has fuelled healthy competition among telecoms service providers in the country, but the acceptance of the service by consumers is still very low. However, findings of the study indicated that with increased awareness about the service and its benefits, and with the erasure of the discouraging conditions that accompany the porting process, more consumers will use the service. Almost all the respondents are using more than a GSM with majority of them using GSM with dual SIM card facility and the reason(s) for it is due to poor service from GSM providers, relocation of users, due to high tariff from some GSM providers, because of browsing data either on modem on another GSM. Finally, it is recommended that GSM service providers should improve their service, equipment and packages to prevent flowing out of their service.