Should i upgrade my iPhone 4S to iOS 8? - 1

I have an iPhone 4S, unlocked and I don't use this phone regularly. I have an overseas phone carrier SIM card installed with overseas phone number and I only use the phone for banking purposes, because my bank overseas sends password to access my account info to my overseas number only. They won't send it to US phone number. So I keep iPhone 4S for that purpose. I have been using the phone and overseas SIM card with no problem, when I'm not using the phone I just turn it off or put in Airplane mode.

Now i get a message about upgrading the iPhone 4S to iOS 8, do i need to do this? I'm affraid that if i do an update what if I can't use my overseas SIM card? Would that happen?

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