Does the wife return to her husband if he divorced her with a revocable divorce by saying "You are divorced," and then took her back with a text message on his mobile phone saying "I have taken you back" before the expiration of her waiting period?

If the marriage contract (Nikah (definition)) is concluded with its pillars, it is valid, and its documentation is not a condition. The phrase "You are divorced" is an explicit divorce, so if the husband says it to his wife, the divorce occurs. If the divorce is not the third and final one, the husband may take his wife back before the expiry of her 'iddah (definition) (waiting period), and this can be done in writing, such as drafting a message via phone.