When I worked on a 700 number service, the number was 700-555-1212 to find out about your IXC. This was over 10 years ago. However, when I just tired it in Salem, NH it did not work, but you may want to try this alternative number.
On Verizon's site, they indicate the 700-555-4141 number so it probably has changed in that time. This number did not work either for me.
Some info on 700 numbers (fromArea 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 local exchange carrier processing the call routes it to the presubscribed interexchange carrier, unless the caller has overridden presubscription by dialing 101XXXX before the number. Thus each interexchange carrier has access to all 7.92 million 700 numbers. 700 numbers are different from all other North American Numbering Plan numbers because the destinations are not unique, and, in fact, depend on the network the caller has selected.