Unfortunately, our medical system is more convoluted than that. PP gives referrals so you can make an appointment to get a mammogram but not mammograms themselves. This is not unique to PP, GP's or OBGYN's you see provide the exact same referral and not the mammogram itself. You can't just call up a place that does mammograms and say "Hey I need one".

For example, I hurt my foot last year and went to see an orthopedist. From there, I had to get an MRI at a separate facility with their referral...they don't do MRI's at the sports medicine clinic. Even if I knew I needed that MRI, I wouldn't be able to circumvent seeing the doctor first because they have to refer me to get the procedure.

The thing about PP & mammograms...PP provides affordable well-woman exams. Many women use them as their primary care which is the only way they can get that mammogram referral. The alternative would be visiting a private clinic which could be extremely expensive if they are uninsured. Without PP services, those women may just go without and then it's too late.

TL;DR: You need a doctor referral to get a mammogram and PP provides that service at an affordable cost to many women. So your phone book analogy doesn't hold up.