Does divorce occur with the statement, "If you took it, then you are divorced," if it is later revealed that she was the one who took the mobile phone, and what are the consequences for the wife for her action?

The majority of scholars—including the four madhahib (schools of thought)—hold the view that a conditional divorce takes effect if the condition upon which it was suspended is met, and this is the preponderant opinion. So, if you suspended your wife's divorce upon her taking her phone, and it is confirmed that she took it, then the divorce has occurred according to the majority of scholars. Shaykh al-Islam Ibn Taymiyyah, however, held the view that an expiation for an oath (kaffarat yamin (definition)) is required if divorce was not intended.

If the divorce has taken effect—which is the preponderant opinion—then you may take her back (raj'a) before the expiration of her 'iddah (definition), provided this divorce does not complete the three divorces. Her 'iddah is calculated by her purification from the third menstrual cycle after the divorce, or by the passage of three months if she does not menstruate, or by giving birth if she is pregnant.

As for your wife swearing by Allah that she did not take the mobile phone, some scholars hold that an expiation for an oath is obligatory upon her, and this is the more cautious approach.