I recently ported my number onto a prepaid plan and popped the SIM card into my Pixel 2, on Android Pie. After a few minutes I was able to call and use data so I happily left the store. Later that day, I found out that I could not send or receive text messages. I called customer service and the computer assistant solutions did not help. I called customer service again and pressed buttons until I get to a tech support person. Their suggestions were not helpful either. I got suggestions like power cycling the device, removing / replacing the SIM card, and resetting the Wi-Fi, mobile, & Bluetooth settings. None of that stuff worked. At the end of the call, they suggested I give it at least 24 hours to complete the porting process. If that didn't work, a factory reset would be the next step.
I waited 24 hours and that didn't work. Decided that the factory reset would be too much trouble.
It turns out that my Pixel 2 was using the Advanced Calling feature and once I disabled this, text messages started flowing.
Go to Settings > Network & Internet > Mobile network > Advanced > Advanced Calling