Why does my iphone 4 hang up by itself everytime?

I recently updated my iphone to ios 7 & Since then, Every time I'm on the phone, at exactly 1 hour and 30 minutes it hangs up by itself.
I'm just so irritated because I'd be talking and it just automatically hangs up on me.
Anyone else having this issue? Does anyone know why it does this & Is there anyway I can fix it?

No it doesn't
This is a network feature to safeguard your time

Say you accidentally dial a number and it were NOT there your time could run and run OUT or you get a big bill

On a Saturday I put my phone on silent as I went to the cinema and DID not lock the keypad as i should have done

It gets worse I put phone was in my backpack so no lights if a text/ call came through

It dialled the last number [ which should have as a Saturday NOT connected] but that OFFICE after call connects you pick an option 1 2, 3. This should have been OFF so timed out after dialling
BUT call connects & 2 hours later disconnects

I explained and a gesture gave me back well under 1/2 the call cost or 41% of the money back as a gesture of good will

In the UK I know TWO networks that both have 2 hours hence my answer

Some networks have it mine is 2 hours, NOT IOS 7, NOT Apple but the Network you are on

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