Is there any way I can restrict my iphone from using the internet? Basically all apps should be blocked from using the internet, in one go, but I should still have the provision to make/receive phonecalls/messages.
Turn off Wi-Fi (in Settings > Wi-Fi), and turn off Mobile Data (in Settings > General > Network).
This will keep the phone off the internet, but still allow phone calls and texts. (iMessage and MMS won't work, however).