EnGenius Technologies DURAFON-SIP System 900 Mhz Radio Frequency, 10-Handset Landline Telephone Up to 12 floors in-building penetration Up to 250,000 sq. Ft. Of facility coverage Up to 3,000 acres of property, open land coverage Base supports up to four concurrent calls. Operating temperature: 32  122F (0  50C) Works with any ip-pbx that supports standard SIP protocols