NEC 110 - Manual (Page 13)

The only authorized headsets that may be utilized with the RNP2400
are those obtainable from SpectraLink or it’s reseller partners.
The measured SAR of the RNP2400 Wireless Telephone is
0.166W/kG @ 2462 MHz (head) 0.0162W/kG @ 2412 MHz (body).
CAUTION
Phone Operation Normal Position: Hold the phone as you would
any other telephone, with the earpiece to your ear and speak into
the microphone. The internal antenna is then positioned properly.