A normal phone doesn’t have AGPS download ephemeris (edit:they may today, I haven’t looked into it for a while), doesn’t have Google Services tracking everything, or third party apps phoning home.
I’d say by default a smartphone is way worse, it has fsr more data collection by default, even without an account. Every data point a feature phone has, a smartphone has, plus more.
Voice calls and SMS use the exact same infrastructure in exactly the same way on both types of phones.
But it can be mitigated quite a bit on Android by not using an account on it, disabling GPS, wifi, Bluetooth.
They could also debloat it to reduce some of the background nonsense (Universal Android Debloat has a “safe to disable” list). (I’m assuming it’s not an unlocked Pixel or a phone that’s on the Lineage list).
If they don’t care about apps, I’d even add NoRoot Firewall, configure it for always on, and set it to block all network access by default. This would be a Global Pre-Filter using asterisk (*) for both the address and port fields with both Wifi and Cell boxes checked (system apps will still have network access, this only affects users apps on a non-rooted phone).
Other than root or flashing a custom OS (like Lineage or Divest, Graphene if they were lucky enough to get an unlocked Pixel), this is about the best that can be done.