Perhaps for video but not for a purely audio call.
For a purely audio call, Skype gobbling all it wants to, you're looking at somewhere in the region of 40-60MB/hr on the data consumption front.
There is no network in the entire United Kingdom who are going to be troubled by or concerned with that low level of usage, even if ten thousand people are doing it all the time, (high quality internet radio will push the same data volume).
What I'm wondering is if Skype still using P2P how it used to and whether Telefonica are identifying that as file sharing and deliberately slowing it and dropping packets, but I'd need an active connection to test.
That seems much more likely to me.
I'll try getting out later to get a voucher and see if I can diagnose.
EDIT: Just tested on a non-telefonica connection running at 115/32mbps and it used less than 20KB/s throughout the call, so I was a little off. It's 72mb/hr at full pelt, but still well below what should and would trouble any ISP mobile or otherwise, in the United Kingdom.