Hi Shepp.. poss take a look at my thread, UK Debts, Spanish Will…
Basically I have been told by a UK lawyer (with specialism in Spanish law), that if the debt is sold on (to a UK co in your case if you are returning there?)... or the Sp co takes out a European court order… Basically the debt will not go away.
Please take half a day to go to your local consumer office, get all the advice there (which usually will involve writing and sending your complaint to the customer services dept of Movistar by registered post). Then at least you have addressed the matter with them. (They may also give you the telephone regulators address to write to also as Jurdy suggests.)
My understanding is that as long as you deal with the matter they cannot force you to pay (through legal assistance or even the courts - unless of course a court decides in their favour.)
I would hope that if you tell them you are returning to the uK, they will either accept the real amount you owe, or write off the debt.
(I am sure there are 1000,s’ of people who have a complaint agiainst them! I am sure my bill has calls on that I never made - but as it is just a few Euros a month, I chose not to remonstrate with them.)
Good luck. SJ