Non of the major players have any worthwhile international deals. Although it does depend on where you will fly to. The US an Europe can have some deals, anywhere else and you will pay.
O2 did try and lead the way with their standard 4 for 1 txt option, which took 4 txt's from your bundle for 1 txt sent from any international destination. The deal stopped last year when non of the other service providers took the bait and tried to compete. If you still have that deal then it's cheaper to buy your own handset if you fancy an upgrade. (if you haven't upgraded for a while, then it's worthwhile asking in case you unknowingly have it.)
If you want to make calls then use skype or one of the similar alternatives as long as you're in a hot spot, although trawling about a foreign city looking for a hot spot just say hi and ask what's happened over the last few days seems inconvenient.
There are international sims that reduce the cost, however anyone you might like to hear from will have to know the new number and when to dismiss your usual number.
So unless you can get the old O2 deal or actively manage several types of contact and it's not an option to be A.W.O.L then you'll have to budget for a larger phone bill.
Personally I only ever txt (a lot) and from some very obscure places and my bill is never more than £30 per month.
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