Jazztel is not a supported carrier. Facetime uses a silent SMS message to contact Apple's servers and activate using your phone number. All supported carriers will permit this message to pass and will not charge for it.
If your plan does not include international messaging, then the message will not be able to reach Apple's servers in the UK and activate facetime with your phone number. If you DO have a plan that allows international messaging, you are going to be charged for those messages.
To avoid problems and charges, switch to a supported carrier. There are 4 supported carriers in Spain: http://support.apple.com/kb/HT1937?viewlocale=en_US&locale=en_US