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“Originally posted by the spiz
I'm not sure if that's true, because roaming depends on the networks in place in that part of the country.
If the major 4 do cover Northern Ireland then it will be fine.
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They do. The costs will be the same as normal.
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The anomaly in the UK are the Channel Islands. Despite Jersey being 01534 and Guernsey being 01481, they have their own mobile networks, thus meaning that a call from landline or mobile to landline is a national call, but a call to a mobile is international roaming 
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If you call a Jersey, Guernsey or IOM Mobile from the UK, you will be charged appropriate mobile rates. If you call a UK mobile which is roaming in any of these places, it will cost you the same as a normal mobile call, but the person you are calling will have to pay roaming charges.
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NI is different in that it is a proper part of the UK as opposed to the Channel Islands. ”
We only got one2one (t-mobile) in NI a couple of years ago, so coverage may be poor in some places - but I think it is poor in many places throughout the UK. We've also got "3" in some areas too.