When you come to Pamplona, it is highly recommended that you come with a mobile phone that isn't locked to a mobile service provider from your home country.
The most prominent mobile service providers in Spain are Movistar, Orange, Vodafone, Masmóvil and Yoigo.
Each of these companies offer two main services, a post-paid service (you need to have a NIE) and a prepaid service (you do not need to have a NIE). If you are not going to stay longer than a semester or at most for the year, it is highly recommended you to get a prepaid SIM which can be reloaded in many supermarkets.
Some service providers even allow you to reload your balance online.
Usually tariffs includes calls within Spain, unlimited texts to other users and 1-2 GB of internet data.