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Solution 1: Roaming with your MTN and Glo line while travelling abroad will ensure you are always reachable even if you are thousands of miles alway from home.

Two mobile phone network operators in different countries enter into a contract called a roaming agreement. This makes them roaming partners’ so for example, MTN Nigeria customers can make and receive calls, send and receive SMS, browse the internet, send and receive emails or any other service when applicable, in the UK using Vodafone.
Vodafone UK customers can do the same in Nigeria.

As an MTN or Glo customer using either a prepaid or postpaid line, your phone is already set up to be used abroad.
Roaming service is available by default on your MTN line. There is no monthly fee, no connection fee and no-deposit required before you can roam. However, the cost of making and receiving voice calls or sending SMS while roaming abroad depends on the country you are visting.

Solution 2: It might also be a frequency issue. The phone is made for Nigerian frequency of 900/180 MHz spectrum while in the US, they use 850 MHZ and 1900 MHZ in Canada.