Hi @Andrej1299
Telegram will ask on the new device to register with your phone number.
Then it will send a code that you'll have to enter.
So as Bolly said, it's best to set your new phone first.
What I do is set everything using WIFI, install the app and configure everything.
Create a backup or transfer data to your new device (if they both run Focal)
Then when everything is working on the new device, I switch SIM on a weekend, so I have time to mess things around if the mobile network does not work out of the box.
Test everything out and you should be golden.