The SPA phones use a 2.5mm headset jack for headsets.
In the event that your headset has a RJ-9 connector, you can use a 2.5mm to RJ-9 / RJ-22 modular cable to connect your device to a SPA phone.
The TelephonyWare SK-42 [tx00537] has been informally tested to work with the SPA phones.
Here's a direct link to the male 2.5mm to male RJ-9 adaptor cable: http://www.telephonyware.com/telephonyware/products/rj9-25mm-sk42.html
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