Best way to trace calls in US / Canada?

I often receive missed calls from numbers not on my phone contact list / phonebook.

Whats the best way to track them down? I say about 70% of them, I can trace them down easily by typing the number on Google, and people will tag them as spam numbers etc. But there are some that are so new that its not on Google yet and I am curious if its a legitiment call or spam again...

I see there are service available such as 'reverse phone number .com' (something along those lines).
How legit are those? and where do they get all the information from? They even claim they have full report on these things?

Thanks

Do it with Gmail and Skype...

Sorry, i think you misunderstood my question.
I have these 'unknown' 'strangers' (maybe telemarketers) calling my phone.
And i am trying to trace them....

Not the other way around, whereas I cold call people...