The ultra low power mode might not be able to do SMS or phone calls over cellular as it needs large bursts of power to TX, depending on connection quality and t

The ultra low power mode might not be able to do SMS or phone calls over cellular as it needs large bursts of power to TX, depending on connection quality and type (2g vs LTE, range to tower, etc). A capacitor that can store enough charge to send/receive a SMS would allow this to work as the capacitor can be charged slowly, but it would take up valuable room inside the phone (a few mm^3). Some embedded cell/sat radios where size isn't very important already do this to reduce demands on the hosts electrical system (eg: solar powered parking meters that take credit cards).