I spoke to the woman calling and, as Annapolitan said, she's part of a "get out the vote" campaign for the upcoming election. It's not a professional phone bank - she's just an individual trying to drum up votes for her political party and to get others to do the same.

thanks so much I live south of Annapolis and was getting calls from the same number

Finally the mystery is solved.  Someone is calling to ask me to vote for a certain political party in November and asking me to phone bank for them.  So this might have been coming from a phone bank.  It sounded like the call was coming from an echo chamber, that's what the sound quality was like.

Sounds like it's coming from a call center.  This time someone kind of muttered to himself, "Okay..." and there was a loudspeaker message in the background.  Irritating.

No message is ever left, but I get calls frequently from this number.