Is such advertising method even legal?

If the advertising text does not violate any laws in the country where advertising is performed, this advertising method is completely legal.

Where did you get the phone numbers where advertising will be sent to?

We created the database by ourselves and checked it with a parser.

Can you target the group of receivers?

Messages can only be targeted by country and by region(London, Moscow etc).
Users can’t be targeted by age, sex or interests.

How many users are active in Viber?

Viber automatically deletes accounts which are inactive for 90 days, which means that every account we send your advertisement to is active.

How many messages will reach the people?

Every and each one of them. Even if we hit a number which isn’t registered in Viber we will get an alert, the correction will be made and we will resend the failed message.
P.S This error checking process is going on throughout the whole message-sending cycle thus, all the messages you ordered will be sent!