If you send a text or make a call to a US number, you’ll pay whatever you normally pay to send a text or make a call to a US number. It does not matter whether the phone associated with that number is in the US or not at the time.
The recipient of that text or call may have to pay extra fees for receiving it internationally, depending on their carrier and plan. Not all international plans are “at no extra charge”; some offer a reduced per-text/minute charge or a limited number of minutes or other restrictions.
You could also use any number of messaging apps (Messages if you have Apple devices, Android Messages, WhatsApp, Facebook Messenger, WeChat, Allo, Telegram, etc…) to send the messages over a data connection without using text messages at all.
Credit:stackoverflow.com‘