How come my iphone hangs up?

After I talk on the phone for exactly 30 minutes it hangs up and it's starting to annoy me. It is an Iphone 4 btw. Can you guys please help me?
PS I have it lock after minutes if it helps.

Can the memory chip on the iphone be changed?

I have 16 gb on my iphone and i want 32, but i don't want to buy a whole new phone, i know its possible to change the chip, but is it cheap or expensive?

Added (1). I have an iphone 5?

What is the best Tablet for my needs?

I'm looking for a tablet that will be able to do the following:
-Play media files (music, videos,)
-Be able to browse the web, facebook and twitter.
-Be able to support the celtx app for script writing.
-Will have an external keyboard peripheral available.

I'm leaning towards the ipad as I can do all this stuff with my iphone, but want a bigger screen, the thing is I'm worried the ipad mini might be too small and not have any keyboard peripherals that I will be comfoortable writing on.
I'm open to android tablets as well however, just have no knowledge about it.

How does the do not disturb work on my iPhone?

I know what the do not disturb thing is on my iPhone & I know all the things about it. There's just one thing I do not know. When I decide to turn it back on, will the missed calls and notifications come in?