NEC Univerge SV8100 System Hardware Manual page 574

Issue 1.0 UNIVERGE SV8100/SV8300
Headsets that have open style receivers (i.e., Mirage, Duoset and Duopro)
can cause echo problems on telephones. The echo suppression
DT700 Series
and receiver gain of the telephone determines the severity of the echo when
using any headset.
Due to the environment where the telephones or headsets are located,
ambient noise may affect performance. Please contact NEC for the
recommended headset to use with VoIP applications.
8 - 62 Installing SV8100/SV8300 Cordless Telephones