Probably a debt collector. Had the same problem with calls very early in the a.m. made to my work phone. I asked our IT dept. to investigate, and this is what they told me:

"Someone did a forced call back. This is when someone calls you and then hits the mail button. This forces the person they were trying to call to give them a call back and will not go away until you reach them."

Nasty, if you ask me, and as ineffective as all the other methods they've tried thus far, including the bogus "delivering papers to the clerk" threat.

Is your company signed up for Ring Central and use an 800 number?

Received a call from this number at 1:26 in the am EDT -  it did not leave a voice mail.  Why would a telemarketer be calling at that time of the day - when most everybody is asleep?

Called me at 1 am but no one on the phone

Called Sunday at 10:30AM left a 4 min silent msg. Interesting because my business phone is a toll free internet(VOIP) number. I am guessing its an automated telemarketer.

Received a call with no one there.  Also, a call from 712 631-6047  earlier around 4 am and same thing.

jerk phone call at 5 AM

called me, third and soon to be fourth mysterious call to my cell today. If this gets worse I have to change my number :( so annoying.

This number called my work and left over 80 messages last night (7/18/13). Annoying.

Got a call from this number and it hung up after 3 rings so I couldn't even answer it.

Have received a 3 am call and a 4 pm call from this number. Only one ring this afternoon. No message left at 3am or the 4pm call. Now I know it's a scammer.

412-888-0749
Called my cell at 11:00pm. And left about 3 min empty viice massage.
Same thing happend to number 435-503-9119 on the same day.

Sunday morning call to cellphone, about 11 am- they don't even give you a chance to answer. This is the third fast-hangup telemarketer call today. I am now blocking all three, but I know it's a losing battle.

Called at 5 AM!!  -- close to another call in another area code.  Same as above.. "not available to take calls...")  kinda like me at 5 AM...

Not a familiar area code. No one there when I picked up.

Called my cell. Not the first time. No message left. I know no one in this area code so probably a telemarketer.

Received a phone call and did not answer it, but ended up with an empty/silent 3 minute voice mail

Received a phone call just now and they didnt say anything

Received a call from this number on Monday Morning, July 8th, before time to get up. did not answer when I saw it was from PA.  Have received calls from this number before. No message was left. Have tried to call back and the recording says, "The person you have dialed is not able to receive calls at this time".