700 numbers are a unique curiosity in the telecom world. Few understand them, and they are not widely used.
The area code, 700, was assigned in 1983 on the eve of the introduction of long distance competition in the US. The intent was that interexchange carriers could use 700 numbers to implement new services quickly.
When a 700 number is dialed, the LEC routes it to the pre-subscribed interexchange carrier, unless the caller has overridden pre-subscription by forcing it to a particular IXC by dialing 101XXXX before the number (XXXX is the "PIC Code" of the IXC).
500 and 700 numbers are not portable (to other carriers), that no other carrier offers 500 or 700 service, that the only available alternative appears to be toll free service (800, 888, and 877 access codes), and that, unlike 500 and 700 service, toll free service does not permit instantaneous re-routing of calls or enable the subscriber to pay only for calls from selected persons, but requires the subscriber to pay for all calls