switched phone and carriers and now can't use iMessage

Hello and welcome to Apple Support Communities, lauri248,

We understand you're unable to use iMessage after switching your device and your carrier. We'd be happy to provide information to help you address this.

The steps here may help you solve any issues with iMessage since you've recently switched to a new device:

If you can't send or receive messages on your iPhone or iPad - Apple Support

"If you have issues with Messages after you set up a new device

If you experience issues like conversations in Messages showing up as separate threads or sent messages appearing as green message bubbles instead of blue message bubbles when you set up a new device, update your settings using the following steps:

1. Update your device to the latest version of iOS or iPadOS if needed.
2. In the Settings app, tap Cellular. Make sure that your phone line is turned on. If you use multiple SIMs, make sure that the phone number you want to use is selected and turned on.
3. In the Settings app, tap Messages. Turn iMessage off and then back on.
4. Tap Send & Receive.
5. Tap the phone number that you want to use with Messages.

If you have issues when you try to receive a FaceTime call after you set up a new device, you can also update your FaceTime settings."

Let us know the results.

Have a great day!