Here is a sample SMS that usually I trust:
The sender is in a text format and there is an information saying the sender does not support replies.
When I tried to login, I received verification code from a different number:
Now, the sender is a normal phone number. I can even reply to the number. There is a few strange characters in the prefix. Also, there is a wrong spelling: "confirm you (should be your) number". This SMS raises my suspicion. While the activation code is working, how can I be sure if the SMS is really send by the provider and not by someone in-the-middle?