MTN recycles number that are in use but no calls made to it.. useless

So get this, for my work I use multiple device and networks, 1 of these is MTN sim, witch after data gets loaded monthly and used, but no calls made. MTN decided to recycle my number and someone else been getting my data ? how the F can data usage not been seen active sim? got told is system function and if don't make calls not seen as active..