Siemens GigaVoice SL30 Micro Accessory Handset for SL3501 and 4000-Series Expandable Phones For Siemens SL3501 and 4000-series expandable phone systems 2.4 GHz frequency-hopping spread spectrum technology Voice operated commands, voice dial, voice announced caller ID Full-duplex speakerphone Long-life battery, up to 7 hours talk time