Can i put my giffgaff and O2 sim an an iphone 4S/5?

Confused

Added (1). Giffagaff works with O2:)

My mian question is can both sim cards go in the phone and can both of them work alongside each other?

Samantha

The phone has to be unlocked or compatible with the sim network. E: G Orange phones work with orange sims only.

Why isn't my GiffGaff PAYG Internet working in my IPhone 3GS? Basically I got an Iphone 3GS and the i put my giffgaff sim in it. I topped it up £15 and got the 400 mins and Unlimited texts and Internet package as i always do. Now when i leave my house (disconnecting the Wi-Fi) my texts and Phone calls are working but my internet isn't connecting. Is there a way to fix this?
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IPhone 4s Wi-Fi with giffgaff sim & no internet connection? I'm planning to get iPhone 4s on pay as you go with the GiffGaff sim, pay as you go. what if I did not pay this month therefore no text/calls/internet for iPhone 4s. Will I still be able to use Wi-Fi? Does it have a limit or whatsoever? Or I can't use it at all as giffgaff does not provide this (does it have to?)?