The FACT about the phones is that phone company will almost always have records and if Casey and Zenaida EVER communicated by phone, there will be a record. And wouldn't be really strange if in 2-4 years of using a babysitter, there was never ANY record of communication between the mom and the sitter!?
The other thing is - if you get a "private" phone call during the month and your bill is itemized, the number will show up on the bill.