Samsung android phones

I have the updated samsung galaxy s3 mini GT handset.

For the past few months my phone has shown signs of "not registerd on network" even when i have good signal....this can happen at anytime and really getting more often now and ive just sent my flat mate to tesco tonight to get me a cheap mobile to use.

Its not only annoying but i rely on my phone to recieve shifts in the hospital as part of the agency and i wake up to voicemails now as they cannot get thru to me so i end up missing out on work/money.

Is it a network issue or do the galaxy phones have a software issue??

Ive tried ringing vodafone but i hate it as i get a indian who i cannot understand reading from a script telling me to take sim and battery out and put back in....its pathetic.

Any nerds on here shed some light please?
Ive downloaded the app where it can reset my signal it seems to help for now.

I bought the phone on contract from phones 4 u in plymouth