Firstly, you might want to read this: Using your phone abroad | Pay Monthly | Help & Support | O2
If you're travelling within the Europe Zone , your phone will work, and be charged, much the same as the UK - but read the article that I linked to in the paragraph above.
Outside the Europe Zone, things get expensive very quickly, so frankly the best advice is to buy a local SIM card for your (unlocked) phone. If you don't wish to do that, then at least turn off your data, and use wifi where available - see below (from O2 website)
What can I do to avoid any data roaming charges?
To avoid data roaming charges you can switch your data roaming off when you're abroad, as apps on your phone might connect to the internet to update or get information even when you're not using them. Switching data roaming off ensures you won't get charged. For help with this, go to our interactive device guide. When data roaming is switched off you'll still have the option to connect to local Wi-Fi hotspots to get onto the internet. Please note some Wi-Fi hotspots in your chosen destination may require payment so we recommend you enquire beforehand.
For more information on what data is and what uses it up, check out our Understanding Data article.
edited to correct spelling
Using Pixel 4 ( O2 + 1pMobile) , Samsung E1182 ( geosim + dtac)
