To make a local call from any dormline, call 90-0 and say 
"I'd like to call xxx-xxxx"

To make a local call from any centrex phone, call 190-0 and say
"I'd like to call xxx-xxxx"

Long distance calls can be made free from many MIT phones
by dialing 4-xxxxx-1-aaa-nnn-nnnn
The aaa-nnn-nnnn is obviously the phone number.
The xxxxx is the MIT Account number to which to bill the call.
The phone will accept invalid account numbers, such as 99999 or 44444
(I don't know that these are invalid, but ...)

Sample phones from which you can do this:
The one in 4-035
The one in 1-3xx (i.e. lecture hall under bldg.1 pyramid)
Many many random offices around the 'tute (i.e. phone closets near
many offices!).

This is, of course, of questionable morality and unquestionable
illegality.  I strongly urge you not to call anyone familiar
long distance, since they might be asked who called them...

[The thought that occurs to me is something like, hey...I've got
a good thing.  Why tell everybody and fuck it up?  But then again,
that's what free exchange of information is all about... :-) :-)]

NB: There is some way to get real long distance free in the real world.
It involves an automated way of establishing conference calls..  It
seems that a person answering a payphone can authorize charges for a
collect conference call...I don't know all the details, in fact,
I know nothing other than what is in this paragraph.

NNB: I haven't tried calling 0 from a payphone and asking for a local
number.  I suppose I should try...

good luck.