I just recently did a insurance exchange with T-mobile but I'm confused.....?

I recently called T-mobile ON 6/9/2011 after months of my "My touch 3G" acting so slow,not taking pictures, and just taking 5-10 minutes to power back up.......when I called T-mobile to do the trouble shooting and the whole 9 they told me I could do a insurance exchange for a "LG optimus" and it should be here ( 6-2011 )....but just randomly the phone started taking pictures and acting a little better but do I return the "LG optimus" or keep the New phone and let T-mobile think I did insurance fraud and charge it to my account....WHAT SHOULD I DO !!!!