Here's a possibility. We know that Penny and her father are rich business owners. Maybe the Dharma project was either started or funded by Penny's father. So when the signal jammer was turned off, it re-established a direct connection to Penny's father ( now hers) office. The reason why she was able to respond so quickly is because of the electromagnetic pulse picked up at the artic station. (Side note: Hurley paints a picture of an eskimo and an igloo which could mean that the station is one of the reason why they are recsued) She probably scanned all frequencies and had an open channel on all of them. She thinks that Dharma island was abandoned but Desmond could of found his way there and figure out how to get a message to her. This is why food is continually delivered to the island also. She hopes he gets there and has enough food to eat