A Pi uses 2 - 10 times the power a typical smartphone uses. A better solution may be something like an ESP32 which is designed for IOT applications and can easily average a handful of milliwatts of power. The trick with any of these devices is to be able to put them to sleep most of the time and only wake to get some work done. ESP32 type devices make that easy. Phone network connection isn't baked into ESP32 devices, but it's easy to add a suitable modem.