left no message. Then when i return the call, there's a "not in service" message. Huh, what is this?

It is "The Mortgage Broker" - they are a recording (without a person
to ask if you want the recording first -- illegal), and they are
calling cellphone number blocks (also illegal).
Let's do everything we can to get these people put out of business.

received multiple, but only partially recorded voice mails, seemed to be about being eligible to reduce interest rate.  Caller says "press 1 to speak to representative".  Anything "inviting" you to press a number could probably be construed as consent for whatever else.

returned this call and it asked me to enter my phone number to be removed from their list.