If you have a cell phone you can take it with you when traveling to another country and you can place or receive calls without changing its SIM card.
Cell phone carriers from most countries have mutual roaming agreements that basically allow you to use their wireless network when traveling abroad. Generally you do not need to do anything - the foreign carrier will take over so to speak and you will see another carrier name on your cell phone's screen once you are in another country. What you do have to do is call your wireless carrier before leaving your country and activate your roaming service. This may not be a default feature (it involves extra charges).
Back in the early days of mobile telephony we had two main technologies used with cellular phones: GSM and CDMA. A CDMA phone didn't work on a GSM network (the opposite was also true). This used to be an issue when traveling overseas, especially for US subscribers to Verizon, US Cellular and Sprint which all used CDMA - GSM was the more widely used technology globally.
Fast forward to 2025, most providers in the US and internationally have or are in the process of phasing out 3G networks and 4G is basically the present day technology used all over the world with 5G slowly replacing it. What this means is that no longer CDMA vs GSM is an issue. What could cause trouble are (very) old wireless devices not working on 4G networks.
Note that you can not dial the numbers in your Contact list as they are - you must add/drop some digits. Use these international dialing tools to find the codes you must dial.
To make local/national calls within the country you are traveling to just dial the numbers locals dial without adding any international codes. In other words, while in another country your cell phone becomes a local phone for all outgoing calls.
One other thing you can do, especially if you are a frequent international traveler, is to store all your contacts using +country code - national subscriber number. This way, when you are abroad, your international calls will connect without adding any extra digits, whereas dialing domestic calls will also work this way in most cases. For instance, US numbers would be stored using +1 xxx xxx xxxx, UK numbers +44 xx xxxx xxxx and so on.
While traveling abroad your cell phone number does not change for incoming calls, no matter where they come from. In other words, people from your home country will dial your number without adding any extra international codes whereas people dialing your number from abroad, including your travel companions, will have to use your full international number including your home country code.
You do, the person traveling to another country. Make sure you inquire about rates and special international traveling offers with your wireless provider BEFORE you leave.