How do I make sure I'm not charged by my mobile carrier? [closed]

I'm currently in a different country from the one where I have mobile phone service. I turned the phone to airplane mode and only connect to the outside world via the Internet. There are many places, including my carrier's own support pages, where they say this will ensure that you receive no surprise charges on your next bill.

I decided that I need mobile data access while I'm here, so I've purchased a SIM card. I've replaced the SIM card in my phone with the new one purchased here and have activated it. It seems to be working well. I have received calls and texts with the new phone number provided with the new SiM card. I have also been able to use mobile data.

Here's the strange part: I've been receiving text messages from people in my home country who are sending the message to my home number, and while not on wifi. How is this possible? Am I going to be charged for this?

Exact details:

• My home country is the USA.
• I'm currently in Australia.
• My home carrier is Verizon.
• My new SIM card says Optus.
• My Optus plan is prepaid, unlimited minutes and texts.
• Verizon has a WIFI calling and text which works anywhere in the world without added charge.
• My phone gives a persistent notification that "This is not a Verizon SIM card."
• My phone is a Samsung Galaxy S8+.