[daniel_j]

When searching for a job is the best example. Though these are usually not hidden numbers and for me, if a mobile is calling it's either a wrong number or work related. Spam seems to always a landline.

[frosted-flakes]

In North America, all phones, landline or cell, have the same number format: 1 [phone], where 222 is the area code.

[daniel_j]

Oh that's interesting, here in Australia mobile numbers are always 04xx xxx xxx (though according to wikipedia, 5 is also valid but i've not seen it)