How can you use your USA cellphone overseas? The answer is both easy and difficult, and depends on your length of stay, budget, convenience, and patience. Here is what I have learned while traveling in Spain.
The first issue is the length of visit to another country. Your main choice is buying data through your own domestic carrier, or access to the foreign country's carrier. With Verizon and other US companies, it's $20 a day to keep your current number but have it activated and roaming in another country, without very much data. If you are traveling for less than 5 days or so, or your work requires you have access to your number, then this is a decent choice. It is easy and convenient. Your domestic phone number will ring your phone, and you have access from the moment you step off the plane and into customs.
If you opt to not use your domestic carrier for overseas access, then you will save a significant amount of money but you will sacrifice convenience. You will need to purchase a SIM card somewhere, and this isn't always as easy as it sounds (there were no such shops in the Madrid airport, for example). The SIM card in your phone is what allows you to access other countries and their networks. SIMs are universal, but not all phones can use all SIMs and networks. Smartphones within the last few years all have the capability of using the major cellular network bands, but double-check your device just in case. The other issue is whether the SIM is locked, with hardware or software, through your provider. This could be because you have a contract on your phone and you have not fulfilled that contract completely, or for other reasons. Check with your carrier to see if your phone is unlocked. Most phones and plans in recent years have been unlocked. Otherwise, your phone won't accept the foreign SIM.
Buying and installing a foreign SIM card gives you a local phone number and cheap data plans. My first data plan with Movistar was €20 for 3 GB of data. Through taking a lot of photos that I uploaded and shared, I burned through this is ONE WEEK! These pre-paid SIM cards do not allow you to easily or cheaply top-up your data plan; you have to wait the whole month to renew it (it is really a month-to-month contract, in that regard). Orange had a summer promotional of 9 GB for the same price, so I bought a new SIM card through them (with a new Spanish number), and I turned off all data backup functions through cellular data.
One issue is that in many European countries, incoming texts and calls are always free, if you have a SIM for their country. So people can call and text you for free, but you may not be able to call or text them back. In the USA we pay for both incoming and outgoing calls and texts (well played, cell phone carriers). But the issue has become fairly moot in the USA because most current plans include unlimited domestic calls and texts; our carriers want you to buy data plans, and then they give you the rest. This is not true in Europe, at least for pre-paid SIM cards, where you put money on your account for outgoing texts and calls. This pre-paid amount is separate from your data plan. I currently have no money for calls and texts, which almost works. You may want to call a place about their opening hours, for example, and I cannot do that. WiFi calls through Google Voice want you to have phone access. Texts through Apple Messages (iOS device to iOS device) on an iPhone seems to assume a cell phone number and access for final delivery, but not on an iPad that is not sharing that number. For fear of being charged international rates somehow, I disconnected my iPhone with Spanish SIM from my iPad.
If you want to save money, don't need access to that number specifically, or are traveling for more than a week or so, then buying a foreign SIM is the way to go. It does mean some aggravation, though. The SIM card and company will assume you understand the language, and will text you offers and information that you may not always understand. My two visits to store providers were difficult because my Spanish is fairly basic, as was the employees' English. Cell phone access and terms are difficult in native languages, so consider how tricky it is in translation for non-fluent speakers!
Another strategy is to just use your home cell service once a week or so. Pay the $20 so you can access your messages and voicemail for one day, then turn off cellular data and depend on WiFi for the rest of the week. Apps like Skype, Google Voice, and Facebook Messenger allow you to call and text to some degree without a SIM card. But some apps (Telegram, Signal, WhatsApp) use a cell phone number to identify and find you, and need an active SIM card to make the last mile delivery. This caused some irritation for me, because I signed up for Signal in the USA to use over here, but it doesn't have a way to migrate to a new SIM card; you have to create a new Signal account with a new SIM card. Since I purchased two SIM cards I had to do this twice. It is an easy process, but your contacts don't have your new information so you have to establish the connection again. This is an issue for WhatsApp as well, which uses your phone's contact information to find friends. If you change your SIM and number but people don't have that information, you can't see each other. It's very strange that such high tech telephony can fail in such simple ways. In this way, Facebook Messenger is the most superior, because it doesn't require a phone number to find you, and it has mobile, iPad, and computer sync.
Another option would be to forego cell phone service at all. It's possible. Your phone's GPS will still work and show you where you are on the map, but you can't use the routing services. If you had decent WiFi and used Skype, you could get by without cell service, but it would be weird in today's age to not be able to call or text while out and about. You couldn’t use Google Maps or other apps for directions.
We live in an age of marvels, but also strange irritations.