How to fix an iphone 4 when it constantly says no service?

I recently bought an i phone 4 from a friend in Spain the phone is on the o2 network and my sim also is, so i thought they would work together the phone has been jailbroken but i don't know how long ago and i have tried things like switching it on and of,
pressing both the home button and the lock button at the same time until the power goes off,
switching airoplane mode on and off
the phone also has cydia so i was wondering if you could do anything with that

Try taking the SIM card out and putting it back it, that usually does the trick for me.

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