Checklist

• The error is in the library's code, and not in my own.
• I have searched for this issue before posting it and there isn't a duplicate.
• I ran pip install -U https://github.com/LonamiWebs/Telethon/archive/master.zip and triggered the bug in the latest version.

Code that causes the issue

self.telethon_client = TelegramClient(
            StringSession(),
            self.device_data.api_id,
            self.device_data.api_hash,
            proxy=self.proxy_data.tg_proxy,
            device_model=self.device_data.device_model,
            app_version=self.device_data.app_version,
            system_version=self.device_data.system_version,
        )
        self.telethon_client.start(
            first_name=first_name,
            last_name=last_name,
            phone=lambda: phone_number,
            force_sms=True,
            max_attempts=2,
        )
...

For a long time everything worked fine, but yesterday we ran into a problem. SMS messages stopped being sent from telegram. At all.
We used various proxies, api_id/api_hash, phone numbers (mostly +7 and a little +1).
There is no error or warning. Everything is as usual. But telegram just doesn't send SMS.

We tried to send the SMS code immediately using the send_code_request method. The result is the same.