This number is with Nationstar. I have a mortgage with them and have a payment plan with them since I was short one month. They call because you are behind, even if you have a payment plan set, because their automated system doesn't recognize the payment agreement. The automated system only recognizes that you are behind.
Nation Star as well as First National both have a grace period that lasts until the 16th of the month, but NStar keeps calling anyway around the 6th or 7th on my cell. I called them to confirm this and they agreed that the 16th was the end of the grace period, but didn't explain why I was getting the phone calls.
Just so you know...If you had a mortgage with Bank of America, they did sell some of their mortgages (including mine) to NStar/Champion. This may be the situation for you, as well. Of course, all of their paperwork was mailed late!