My samsung f480 has limited service even though there is full signal

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if you have a limited service it means no signal or no network service available, same thing really

That means, your service provider is active in the current area.

even though you have switched to a pre-pay ...that is a data phone and most companies don't tell you that a data plan is needed 4 the phone to operate correctly.data plans run between 19 and 50 dollars...depends on your provider..

it indicates that your signal strength is weak
Check for the signal from your service provider

Try to call your service provider to check the network configuration of your phone to refresh your account..

I just called my service provider and they were able to fix it. Just gotta wait 4hrs then turn it off, wait a few seconds then turn it on. heaps quick they knew what i was talking about straight away

Normally, your service needs to be specifically enabled to make video calls (especially if this your first 3G phone and you've just used your SIM card from your old 2G phone). If this is the case ring your providers customer service and they should be able to enable it there and then.
Also, both you and the person you're ringing have to be in a 3G service area (often more limited than the general "voice call" coverage area).

TYPE THE CODE OF SAMSUNG
*2767*2878# OR
*2767*3855#

Did you buy the phone brand new, or used? Try doing a master reset on the phone.