Sim Card - UK and Spain

There's no easy answer, as you've seen in your research, although folks around here have preferences for brands and procedures. You might want to just scroll back through the topic for a few months' worth of posts. I also suggest you drop into your carrier's store and ask to talk to someone who understands your provider's and your phone's many international calling options; eSIMs are all the rage these days.

Some of us use a Vodafone UK Sim card. You can use it in most European countries but I don't think you can call to the US. We use Whatsapp for those calls. They're free but both parties have to be signed up with Whatsapp.

With the Vodafone SIM you can buy a Big Bundle which gives you unlimited talk and text and it was recently increased to 7GB of data for 30 days. We brought our SIM card on Amazon or Ebay a few years ago. At that time it was $5 but I don't know how much it is now. The Bundle is 10GBP which is about $13 for 30 days. We have used this SIM in many countries. This would solve your issue of getting a SIM to use in more than one country and your European phone number doesn't change.

We find this SIM is very convenient and Vodafone customer service has been helpful in setting up the Bundle from home before we leave on a trip. If you are interested, CWsocial has a post with a lot more information.

One question: how many days in Spain, how many days in UK? That information will help us to reply. If most of the time is in the UK, you may want to get a UK Sim plan and use roaming in Spain.

The Orange Holiday Plan offered in Spain will include 100 minutes of calls to the US, as well as data and roaming in Europe and the UK.

https://tiendaonline.orange.es/tarifas/movil/llamadas-internacionales/Holidays-Pass-15

Depending on your phone, you can buy an actual Sim or eSim. Orange is a major service provider in Spain. When you roam in the UK, the mobile operator EE will provide the service. Orange has stores in Spain where you can purchase this Sim plan. It can also be topped up.

You just need to look into the details of the plan you buy. The EU imposed essentially free roaming a number of years ago, but with Brexit, some companies have started to exclude the UK from coverage, since it is now voluntary.

My hunch is that it would be easier to find a SIM plan that includes the UK and EU in the UK, than in Spain, but worth a look. Vodaphone would be a good place to start, might be you have to pay a bit more for a plan that includes the UK, same with the ability to make calls to the US, if you go to a store, they usually can help with a plan, and install the SIM, get the phone working.

tigerjzee, if you need unlimited calling and texting to the US, you can get both for free using the Google Voice app. You can call regular US phone numbers for free, even landlines. The person receiving the calls or texts does not need the Google Voice app - only you do. Just install Google Voice before you leave home and try it out. You can use it on WiFi - don't need a European SIM card at all to make calls or send texts. Calling non-US numbers is not free, though - costs a few cents per minute.

The SIM card gives you mobile data anywhere, whether you have WiFI or not, and of course allows you to make local calls within Europe. If you only need to call/text home to the US and can manage doing that while you have WiFi, you don't need a SIM card at all. Just keep you US phone in Airplane Mode then turn on WiFi.

I don't know post-Brexit whether a Spanish SIM would cover the UK or vice versa. But if not...just buy another SIM card in the next country. They don't cost much.

As of June 8, 2022, the Vodafone UK SIM will become both more complicated and more expensive when roaming in Europe, outside the UK.

Please read my latest post with some details of their post-Brexit changes:
https://community.ricksteves.com/travel-forum/tech-tips/vodafone-uk-sim-card-post-brexit-pricing-and-complications