FAQAdd to FAQ (Coming Soon)
There's a brief moment in the film where McKittrick's programming team are talking with the government officials about how David did it: somehow a phone line got "crossed" in Sunnyvale, California, the same city where Protovision's headquarters were, which is the same company that David was trying to hack into to find out more about their new lineup of games coming out soon.
One of McKittrick's team has a quick line where he says "... the phone company screwed up!!" Edit (Coming Soon)
In addition to looking for Protovision, David was looking for other businesses in the area that could be useful in his hacking. As we see, while Jennifer looks on, he finds a bank & Pan American airlines, which he immediately hacks into and makes a reservation for himself and Jennifer on a future flight.
When Jennifer points out that dialing so many long-distance numbers can be expensive, David is unconcerned because he probably already knows how to erase the call records in the phone company's system.
Because of the film, the practice became known as "war dialing". Edit (Coming Soon)