Keeps calling but doesn't leave a message

Cell phone rang. No message left.

I called this number back after the only thing left on my voicemail was a "beep." The recording states that they are TRS, a  Business to business marketing company and calling center.  So who knows why they were calling.

For several months have received calls.  I have never picked up as they would never leave a message.  Several times a week for several months.

Same here. Caller never leaves a message - just a hangup click.