Skype Not A VOIP Service On 3 X-Series Mobiles

If a large number of your friends and family are on Skype, then a plan such as 3’s X-Series — which includes a fixed number of Skype-to-Skype calling minutes direct from your mobile each month — can sound tempting. However, as Decoder points out, those free minutes are restricted purely to the official 3-sanctioned Skype mobile client, in large part because Skype on 3 runs as a callback service rather than ‘pure’ VOIP.

That means you can’t use alternate clients such as previously mentioned Fring to consume your free minutes.
That doesn’t make the plans a bad deal, but it’s something to bear in mind if you’re looking for maximum flexibility.