Quick answer: Check the country code, format and actual account first. Reaching code entry still does not mean the application is complete. GetTGAPI can guide the process, but an account problem must be addressed separately.
For help, contact support at gettgapi.com/en/help#contact. Already have an access code? Start at gettgapi.com/en/redeem.
Identify where the error appears
This situation occurs immediately after the number and Next: ERROR, invalid number or failed sending, before code entry. App title is not yet involved, so changing it is not the first check.
If the page is waiting for a code with no notification, see missing codes. If entering a code triggers the error, see matching codes to requests.
Enter the number correctly
Telegram requires international format, with + and the country code. Official number guidance
Check the entire number, not just its last digits. A wrong country code can still look plausible. Usernames, user IDs, Bot IDs and contact names are not phone numbers.
We remove spaces and add a missing +; you must supply the correct country code. Do not include brackets, hyphens, letters or full-width digits. If pasted text fails validation, try typing it manually once.
Correct format can still fail
Valid structure does not establish that the number is registered, currently usable or accepted for this request.
In your Telegram client, check that Settings shows the same number and you can read that account's messages. If you changed numbers recently, use the current number, not the old one.
Unregistered numbers must first register normally through the official client. API assistance does not replace account registration or create a Telegram account from a number.
INVALID is not BANNED
A generic website ERROR is not evidence of a ban. Explicit client errors such as PHONE_NUMBER_BANNED have specific meanings; PHONE_NUMBER_UNOCCUPIED indicates an unused number. Keep these client errors separate from a generic web message. auth.sendCode errors, auth.signIn errors
If the official client cannot register or sign in either, use Telegram support. Application assistance cannot lift bans or make an unusable number verifiable.
Next step
An accepted number and code-entry page mean phone submission passed, not that API credentials exist. Completion requires ID and Hash.
If your number works but you need help, contact support and use the application page with an access code. Saying the error occurs after phone submission is more useful than simply saying it does not work.