Here is a map where the majority of states allow you to record phone calls with only one party (you) knowing that the call is recorded. https://deadlyclear.wordpress.com/2013/04/15/is-it-legal-to-record-the-banks-phone-call-or-conversation/

It’s much easier to write your follow-up letter if you have a recorded conversation to work from – and most attorneys advise to write the servicer a follow up letter, especially when a recording might not be admissible in court. Keep all your correspondence, including statements organized. If you are unorganized in everything else in your life, make your mortgage loan your priority in organization. The correspondence you keep, may save your home.