Texting from a disabled iPhone?

I recently reported my iPhone 8 plus stolen due to my ex not giving it back after a breakup. I used the at&t app to report it stolen, and to disable the phone. However, this was about 12 hours ago, and I got a text message, actually, an iMessage from the phone number/from my ex. Is this possible? I was informed by AT&T that the phone was unusable. If I call it, the phone says "not accepting calls". But I thought for sure, the entire phone was disabled, not even the use of Wi-Fi was possible. Please give any information.

All that does is turn off phone service. It still runs over Wi-Fi. To fully shut down the phone you need to put it in Lost Mode through Find My iPhone. If you don't have that, at least change all the passwords your phone knows.