Why is the login code sent inside Telegram instead of by SMS?

If your account was recently active on another device, the new login code may be delivered to the official Telegram chat on that signed-in device instead of by SMS.

Open Telegram on the device that is already logged in and check the message from the official verified account.

This is a normal security mechanism that gives priority to devices that have already been verified.

If your phone number has changed, update your account information as soon as possible. Otherwise you may keep access on an old device for a while but lose the ability to recover the account later.