not sure if you have resolved the issue yet but the fault is the SV needs to be upgraded from its current software.

The fault is and knwon by NEC is certain callers in have lower voices than others, and the system doesnt realise the caller in is still there (with a softer voice) and hangs up.

This was an issue with version 5 and 6 of the SV. Upgrade to the latest and it will fix the issue. If it doesnt, NEC recommend a complete default of the SV and start again.