Android VoIP apps can get you an alternative phone service on your mobile for little to no money.
A VoIP app allows you to make calls from your mobile phone. You only pay for the data usage over 3G/4G. Your mobile phone provider may charge you for a local call.
Currently, Android holds 40% of the smartphone market. Part of the platform's dominance has to do with the wide variety of phones and its availability between the major mobile providers. It wasn't until October 14, 2011 that Sprint got a hold of Apple's iPhone, and there is only one to two model iPhones on the market at a time. On the other hand, Android smartphones are sold by every major provider (AT&T, Verizon, Sprint, and T-Mobile) and many different manufacturers.