The "wpcf7_form_tag" hook isn't being executed twice, but CF7 is detecting each of the field names twice, not sure why. But that wasn't the cause for eventually having NULL values...
The phone number wasn't the only thing that was becoming NULL, my entire formidable array was becoming NULL.
My problem was related to the way I obtained my formidable values from the Database. I was creating a new DB connection each time my function ran, which was a big NoNo. Just made my external DB connection a global one, and problem disappeared.
I think it was a speed issue, since there were two functions trying to query the same database table.