I have heard that often when replacement phones are given, they are refurbished phones, not new. And that sometimes, refurbished phones, aren't really refurbished, just handed out to someone else.
I know with tmo, that after 2 or 3 replacements with refurbished phones, and you still have problems, they will then give you a new phone. At least that is what they did for my daughter.
So, I would suggest, that you turn that phone in and try for another one. Since this happened originally with less that 30 days, I would insist on a new phone and not a refurbished one.