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. I was unable to obtain a datasheet for this, however I did manage to get hold of a datasheet for a TNETV1056 which is very similar, I don’t know the exact differences between the two but if you’re interested in hacking the chip the datasheet shows it has a JTAG interface.
You can download the datasheet here.
At the time of writing I had just bought 100 of these phones as a ‘job lot’ off eBay, they cost me £4 each and I am happy to sell them at cost. If you’re interested get in touch, you can either collect from High Wycombe, Buckinghamshire or I will post them at cost. Every phone I sell will be powered up and factory reset beforehand, they are what I would describe as “Class B” in that they may have minor signs of use but are generally in good condition, here’s a photo of one of them…
Upgrading the firmware
To upgrade the firmware you need to plug the phone into a network and power it up. You need to download the firmware from Cisco (it’s freely available) unzip it and put the .bin file onto a webserver. You then need to point your web browser at the phone’s IP address (you can find this from the “Network” menu on the phone). Click the “Admin Login” and “Advanced” buttons in the top right. There will now appear a “Provisioning” menu – click this. About halfway down the page there is a field called “Upgrade Rule:” enter the URL of the firmware you placed on the webserver here then click the “Submit All Changes” button. The phone will now download the firmware and upgrade – this can take several minutes, so be patient!
NOTE: If your phone’s current firmware is below version 7.5.2 then you must upgrade to 7.5.2b before you can upgrade to anything higher.
To save time I have placed the firmware on a webserver you can access, the URLs are:
http://p.telgw.uk/spa/spa50x-30x-7-5-2b.bin
http://p.telgw.uk/spa/spa50x-30x-7-6-2g.bin
Version 7.6.2 being the latest available on the Cisco site at the time of writing. This firmware was dated 2020 and as the devices are EOL (End Of Life) I think it unlikely Cisco will be producing any later versions.