You MAY be able to change your mobile phone to Spanish (to get a Spanish number).  Some phones are locked to a particular provider but it is possible to get them unlocked.

You can buy a new SIM card (€25 including €16 of calls) from Sabeco.  Various networks are available (I use Movistar).

To add extra credit, just go into a phone shop (for example Telefonica in Placa de Alfonzo in Tortosa) and you can add credit straight away. Normally in multiples of €5