Pros
It is a company with a massive footprint (geographically, technologically, etc). Probably any given company with presence in Spain has some sort of contact with Telefonica (gets its telecom services, sells products/services to Telefonica. etc). That offers a huge spectrum of opportunities being both client or provider of them. If you fall in the right unit there are always opportunities to move to some of the countries where the company has operations. Networking (and unfortunately politics) plays a big role in the day-to-day of the company and people use it in every way possible so you got to be in good standing with the key people who have good "hands" in the politics game. Usually if you do your job well people will look for you and you will be well considered, but always pay attention to what is said between the lines and, without stopping doing your job, play your cards well.
Cons
If you do your job but totally disregard the politics of the unit you are in, then you will miss opportunities and, if you do your job well, people will try to use you. Only having a good manager can help you if you are good are your job and people come at you in a "I'll-go-to-this-guy-so-s/he-will-solve-my-problem" way. Not just do your job, pay attention to who you solve problems for or who you do job-related favors for and look for them when you need them. In my opinion salespeople fundamentally drive the units and nothing can be done about it. Even if you are in a Customer Service & Operations unit and it would be obvious that customer satisfaction would be the key, the sales rep will have the last word with the client even when the service is being put in operation and the deal closing is long passed. There are so many outsourcing companies selling services (and here I'm specifically talking about professional services) that if you are a contractor you are pretty much hanging by a thread stability-wise. You could be working in the same unit for 10 years, accruing all the knowledge and from one day to the next a Purchasing rep decides that they want a resource cheaper than what your firm charges for you and next day you are relocated to a different client and the unit has lost 10 years of knowledge. It is worth to mention that the knowledge bases in Telefonica are non-existent or not well managed so the day you leave after 10 years at the company your knowledge leaves with you. It dependes on many factors but you are working on a project and specially if you are contractor you will work long hours.