This simple circuit gives a clear indication when the phone is use - handy for those who still need dial-up for Internet access.
Tiny Real Time Clock (RTC) I2C module - DS1307 AT24C32 TE187
An article on a DS1307 based I2C module which can be used with Arduino programmable microcontrollers. These seem to be sold with non-rechargeable batteries, but charging circuitry for rechargeable batteries, which means some work is needed to improve the battery utilization.