Are all the Sky mobiles that fail to connect all being used in the same geographical area or are they from different parts of the country?
Hmm, do you have any contacts in a different part of the country that use Sky mobile and could make a test call to you?
as I already posted I have DV and use SKY mobile and have no problems with calls
Indeed, which is why I suggested that someone like you would be a good test of the OP's DV. If you were to try calling him & couldn't get through it would point to an issue with his DV. If you could get through it would point to something peculiar to those Sky SIMS or maybe the local mast.