Yes - we went through this recently!! Having been told in the shop when we bought fibre optic that we could suspend the contract while we were in England for the winter, we duly went in to the shop in October to give notice and were told we needed to telephone their customer service on 1004. I did so, only to be told it wasn't possible to suspend on the contract we are on (€62 a month Fusion). We went back to the shop to get some more info on why we had been misled and one of the ladies in there told us we needed to call and ask for the Contrato Basico (basic contract) at €4 a month. I made the call straight away while in the Movistar shop to an English speaking advisor and was told, like you Helenjo, that I needed to send a letter. The shop assistant helped me by speaking to the advisor on the telephone and it was agreed that as long as I had reasonable Spanish, I could answer a series of questions which would be put to me only in Spanish, then this would be recorded and would suffice for their system.
Our secret weapon was that I had my phone on loudspeaker and the shop girl directed me to read the right answer out at the right time - pointing in sequence to my NIE, landline phone number, address etc. which was on my contract paperwork and recent bill. I cannot deny you need some Spanish if only to correctly pronounce alphabet letters and numbers for your NIE and landline. Assuming you're confident at that level it is possible to do it that way.
There then ensued some debate about whether the router must be returned once on the basic contract; the telephone advisor said it must be returned to the shop, while the shop girl hadn't heard of anyone needing to bring back the router if you stick with Contrato Basico of €4 a month. More confusion!!!!
Having got that far, we reconsidered and decided we wouldn't go ahead with the whole rigmarole and are swallowing the cost of all year round service rather than try to save for the months we may not be there - mainly because we do nip back and forth and might as well have TV while there!!
But it IS possible to do it with a helpful Movistar shop assistant and reasonable Spanish!!
Good luck!!