caller unknown

the past few weeks, i have been receiving numerous calls from a blocked or unknown number. sometimes i'm in a meeting and just let it go to voicemail, but i have yet to get a voicemail from the unknown caller. other times when i am free to pick up, the caller immediately hangs up. i've probably gotten around 7~10 calls like this over the past two weeks and i'm not really sure what to think. debt collectors? a stalker? or maybe someone who is just bored?