They did try to contact Cassie. Joe Petito said it on Dr. Phil.
As I say further up on the thread with generic delicacy, there are relatively inexpensive ways and databases to pay in order to get people's phone numbers, including cellular numbers.
IMO, that's a lot of extensive phone numbers for the Petitos/Schmidts not to be able to locate by any means.
4 different Laundries, 5 if we count Cassie's husband, and every single number they dialed for them was wrong or nonfunctional?
Especially since, IME, parents of school-age children such as Cassie still have a landline in addition to a cell phone, just to make sure they don't miss any calls from a school?
Gabby knew the nephews to be able to address them by name, but Joe and Nicole didn't know Cassie and her husband's names?
It's not impossible, but it's highly unlikely, IMO. Plus the fact that thousands of people everywhere were eager to help them find Gabby - how much do you want to bet that some of them offered a wealth of location/locating tips on how to get more addresses/phone numbers?
The more people you try to contact, the less likely it is that all 100% of them fail to receive your calls; and that's just basic probability.