While I am in Spain can I use a Spanish sim card in a cell phone purchased in Italy?
A) The phone is unlocked
B) The SIM card is the correct size. There are 3 sizes of SIM card, "normal" "micro" and "nano". Newer phones use smaller cards.
All mobile phones in Europe are on the same technically standard (GSM) which means the cards are all electrically identical and compatible.
Thank you. That helps
Yes, you can buy a SIM card at the 'Telecor' shop at Puerta de Atocha train Station (since you plan to be there-- answered your other post)
We purchased a Lebara SIM card (in 2015) for my old (unlocked) flip phone @ €10/month, which provided 400 minutes (domestic & International). The salesperson will be able to tell you which SIM card is best for your specific phone.
Here is a map of Atocha's Station. Telecor is labelled #16 on the ground floor (Planta 0):