To make a call on a US or Canadian smartphone when you are in Europe, you must dial:
+34.123.456.789
The “+” tells the system that this is a call to another country. You may physically be in Spain, but your phone still thinks it is at home.
The “34” is the country code, in this case Spain.
The remainder of the string is the local phone number.
Depending on your home cellular plan, this can be expensive. Caveat emptor...
Using my US t-Mobile Simple Choice 55+ plan, data and text out of the US is unlimited and free. T-Mobile only get difficult once I am foreign more than 30 calendar days at a go. When I land at Madrid, my phone just boots up and connects to either Vodafone.es or orange.es.
All voice calls placed while I am physically out of the US cost USD .25 per minute. So a local call in Spain to make or confirm a reservation would cost 25 US cents per minute.
Living in Canada, your cellular provider options are different and may be more limited or expensive. Shop around.
Before swapping SIM cards, I would find out if your phone can make Wi-Fi calls. If it can, you put your phone in airplane mode, turn only Wi-Fi on, and place your phone call as a a VOIP (voice over internet protocol) call. Of course, you need a good Wi-Fi signal to do this.
If using your home cellular provider is either not possible, or is too expensive for you, you can pick up an Orange.es or Vodafone.es SIM on arrival in Spain. Make sure you get generous data and text. Voice is good to have, but less essential, at least IMHO.
Hope this helps.