A VPN pathway was a way of stopping cell phone calls from being monitored. It could take someone about an hour to crack.
When Audrey Raines left CTU during Day 5, she asked Chloe O'Brian to make her calls untraceable. Chloe explained about the VPN pathway to her and set it up on her phone. When Audrey called Chloe from a gas station, she was switched over to Chloe's own VPN to stop anyone monitoring either side of the conversation. ("Day 5: 11:00pm-12:00am")
Later, Chloe set up another VPN pathway to Jack's phone so that CTU couldn't trace their calls, but told him it would only work with the phone he was using. After Henderson was apprehended, his men were monitoring all non-VPN cell traffic, so Jack told Audrey she could only make calls to his phone. ("Day 5: 1:00am-2:00am")