In episode 211, "2πR", Harold mentions that his phone numberr is located within the first 3,000 digits of Pi. Assuming he has a NYC area code, his number is one of the following: 2129021960, 3479131515, 3479775356, 3475741849, 6462080466, 6465757396, 6461515709, 7187110145, 7181921799, 7186494231, 9171536436, 9173637178, 9178608578, 9172874677, 9178249385, or 9298489608.
According to this wiki, "Harold has a voicemail box number @ (917)-285-7362. Note this is not in the first 3000 digits of PI."
If his phone number and voicemail have the same area code, this narrow the options down to: 9171536436, 9173637178, 9178608578, 9172874677, and 9178249385.The first of these numbers does not exist, and calling it returns a generic "call cannot be completed" message. The other four numbers are all in use, but unlisted. Thus they most likely do belong to real people.
Could the first number be his, hidden from the system? Is it one of the four? Could it be something completely different that we have no hope of discovering?