It may not occur to most of MTN's subscribers, but losing cellular signal of your Android smartphone is quite common in congested urban environments, below ground, or in really bad weather, as all of these will reduce your phone's chances of communicating with the cellular tower in your area.
When you want to measure the strength of a given signal, you are actually trying to measure that power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW), which is more commonly referred to dBm or asu. Probably, you will see a negative reading for the signal power of your phone's reception, though - that's how it's supposed to be. Yet some devices may omit the redundant minus sign.
You should have in mind that lower is better, meaning that a signal strength of -80dBm is multiple times stronger than a reading of -95dBm, for example. When it comes to mobile cellular signal, the absolute best you could possibly get is around -50dBm, whereas readings near -120dBm mean that your phone's about to drop its cellular connection anytime now. The lower the number, the better the reception.
Now, in order to see your device's cellular signal strength on most Android smartphones open Settings -> About Phone -> Status-> Network.