SNOM870 - Dialing issue when headset connected

The SNOM ‘community’ forums are now locked and limited to ‘SNOM partners’ so I have have not many places to ask this type of question.

This morning I had a report of one user’s SNOM870 station displaying letters instead of digits when dialling using the keypad. We power-cycled the unit without changing the behaviour. However, when we disconnect the headset then digits are displayed. Has anyone using SNOM870s seen this event before and if so do you know: 1. What caused it? 2. How is it cured?