IPhone 4s problem with connection?

I bought an iPhone 4s from someone yesterday and they showed me it working fine with their sim. So I trimmed my sim down and yey it worked. Phew!
Anyway since then I've found that if I turn my phone on I can get onto the app store and download one or two things then for some reason it will just lose connectivity to it and come up with error MSG 'not connected to iTunes'. If I turn my phone off and back on it will work for a few mins again but then the same?!
This never happened in my last phones n I've used samsungs and htcs. Other aspects of online Like search engine and Facebook work fine through the 3G (as we do no have Wi-Fi currently) its just the app store and apparently iTunes although I haven't tried that yet?
Anyone know what the prob could be?
It couldn't be cos I trimmed the sim could it?

Most likely because you trimmed your SIM card.

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