Just an update - we are now using our lovely 3G touchphones which we got free with our Yoigo contract (6 euros a month, calls at 8 cents a minute). The customer service was excellent; we did it through a booth in a commercial centre, rather than online, and it all went very smoothly.
We transferred our existing Movistar numbers and Movistar rang EVERY HOUR, ON THE HOUR for two days solid, and sent a total of 14 texts, trying to persuade us to stay! They must be losing customers right left and centre.