By definition, all ISDN is BUSINESS, not residential. However, they call it residential when it goes into a house, but it is still a business line.
In Fort Worth Texas we have a flat $57 a month charge (plus long distance, if any) but no extra charge for minutes connected. GTE sounds like a ripoff.
Since ISDN is a business service, check with your local Bell company. They are offering no charge connections, etc., down here all over the place, several times a year.