One thing that springs immediately to mind is that in ATAI, Martha and Jonathan were about to embark on a European vacation to celebrate their wedding anniversary. I'm sure that would have been a number of weeks, and they wouldn't necessarily have been in contact, especially if they were travelling around.
Anywhere they went on holiday, they might rely on Clark's being able to get hold of them in a hurry rather than leaving phone numbers - or, if he's unconscious, he might be the only one who knows where they are.
Mere
