One thing to be aware of is that Caller ID's are not reliable. They are reliable if you call somone from your personal phone (the right number or blank is assured) however large phone users can put what they like in the Caller ID.
Before the OUTRAGE
boils up too far there are sound technical reasons for this.
A large organisation might want to have the corrrect extension number in the outgoing phone call. They get to populate the caller ID field.
A large organisation might want to buy phone lines from more than one supplier and wish to preserve the caller ID irrespective of the Telephone Company that any particular call actually goes out on. They get to populate the caller ID field.
Obviously, it's even more entertaining for calls that at some point go over the internet as normal IP traffic.
Anyway the fact is that users of large numbers of phone lines can put what they like in the caller ID.
It's a matter of the equipment you have on your site rather than the number of lines you have but typically it applies to companies with quite a few lines.
Just something to remember.