Vodafone have a promo on giving you the international calling add on for free, for your entire contract. (Looks like pay monthly only )
It's normally £5 a month and gives you very discounted prices to call abroad compared to their extortionate normal prices.
Doesn't seem to be account specific , and it says on the site very clearly the bundle is free for the duration of your contract or until you opt out, so no charges after the first month.
Text VFINTFREE to 97886 to opt in (different number to the 100GB offer btw)
For example 5p a minute to USA instead of £2
And 15p a minute to Spain instead of £1.50
No limit on the amount of minutes that can be used on the discounted rates.
I know there are cheaper services around, but this is for the people who call their car rental etc abroad once a year for 10 min , and incase you by mistake use your phone to call abroad instead of an app like I do sometimes.
Credit to @gulbinas2004
for the deal.