It breaks surprisingly commonly. You've moved 3-7 times in a few years. I move different customers numbers at many times a month. Around 20% of the time we have issues with fail to fast busy for 8 to 12 hours where no system on the telco side takes a call.

I will say this though, switching mobile goes rather well almost all of the time. Porting numbers on fixed telephone lines is much more apt to fail.