@Arthur15 Neither you nor your guests can respond to an SMS notification from airbnb. Those are only notifications that a message has been sent. It will sometimes have all (if the message is short) or a truncated version of the message if it is long. Both you and your guests have to log-on through the app or desktop, to your accounts, then go to the message stream to write and read messages. BTW you are far from the only person who doesn't understand this, there are posts about this all the time.
Your and your guests' telephone numbers and coded email addresses will appear under your profile photos after a booking has been confirmed, never before. And if you try to put personal info like this into your messages before a booking is confirmed, it will indeed be blocked.
All of this is actually a good thing- it is a security safeguard and it is also a good idea to have exchanges between you and your guests documented through the system, in case there are issues. It's no big deal to text or phone a guest's private number if say, they're driving around and can't find your house and need directions, but anything regarding the booking, house rules, missing items, guests asking for things, etc best to do through the platform.