Called my cell at 11:30 a.m. - I didn't answer and they left no message.

Same... one ring then hangup. I think they are using this strategy to circumvent the do not call list. There are loopholes and I think one of them is if YOU call them, they are allowed to call you and you can't charge them with a violation. They are betting that some number of people will call the number, thinking they got a dropped important call. I would put money on this or something similar because of so many reports of hangups.

This number called, I picked up almost immediately since I had my phone on my desk... and after 10 seconds of me saying 'Hello' and no one responding they hung up. -.-

Telemarketer trying to lower my interest rate on my credit card.  Blocked by Mr. Number

THIS IS A SCAM METHOD designed to sort-of get around the "Do Not Call" list restrictions. They call you for a couple of rings and hang up. Naively you call back, thinking you missed a call from somebody you know, or it's a call from an employer, etc. ... and that gets around the DNC list, since YOU called them at that point. Then they launch into their sales spiel.