These phones are available at very low cost, e.g. on eBay and work great as a VoIP phone. They support up to 4 “lines” or SIP accounts and are very easy to program. For anyone interested in hacking the onboard CPU there is a serial interface available on the PCB – Look for “J3” which is a 5 pin header near the MX T122541 surface mount IC (this chip is a flash memory chip, presumably where the CPU stores it’s firmware).
With the 5 pin header to your left and the MX T122541 IC to your right, the top pin is GND, moving down the next pin is not used, the next one down is RX, then TX, the bottom pin is VCC (+3.3 volts). The CPU sends booting information from this serial interface, connect it up to your PC and set the terminal software to 9600 baud, 8 bits, no parity and 1 stop bit (96008n1). The easiest way to connect this 3.3v serial port to a PC is by using one of the cheap converter boards found on eBay/Amazon/AliExpress, they can be bought for a few pounds, just search for “FT232RL PCB”.
The next chip to the right is labelled WINBOND W9812G6JH-6 and is a 16MB RAM chip. The next chip to the right is labelled TNETV1057ZDW and is a SoC made by Texas Instruments. This is the CPU that runs the firmware for the phone.