I have a bit of a thing for ruggedised telephones like the ones you see on railways and industrial settings. In the UK the main manufacture is Hubbel, formerly Gai Tronics, formely DAC...
Their phones cost a fortune new but occasionally pop up on EBay for reasonable prices, however they're usually in some state of disrepair and spare parts cost a fortune. £200 for a replacement handset for a £600 phone. Yikes! (Incidentally I suspect this is why the end up on EBay, it makes more sense to buy a new one than try to get spares)
For a long time I've wanted to create my own custom internal controller to replace any that are broken, with the added bonus of converting them to SIP. I had some concerns about interfacing the handset / reciever with a soundcard as telephones generally operate at a different impedance to your average speaker and microphone, so I connected one up as a proof of concept.
I chucked a photo up on Twitter and it was suprisingly popular so I thought I'd document the process
New Teams Headset pic.twitter.com/lOA4McTV5f— Matthew Harrold (@MatthewHarrold) July 29, 2021
- CM108 USB Soundcard (~£5)
- Telephone Handset - I used one from a Gai-Tronics commander, but others may work. Unfortunatly I can't guarantee which will be compatible.
- Soldering Iron / Hot Air Rework Station / Solder / Electronics Tools / Wire
Identifying the speaker and microphone
Converting the CM108 soundcard
Connecting the handset to the soundcard
Connecting the soundcard to the computer
Once you're confident you've identified the microphone you can select the output to use the CM108 USB device and playback the audio, slowly increasing the audio until it's at a good volume through the handset. For me 50% was more than loud enough, but I could go as far as 100% without distortion or damage.