Prison Telephone KNZD-27 GSM-C KNZD-27 is a professional telephone for explosion-proof prison telephone. prison phone calls usually need absolute strongest phone. People put money on phone for inmate to make phone calls inside the jail. KNTECH is prossional communication system provider for inmate phone service. KNTECH manufacturing inmate telephone system include IP PBX and inmate phone. We also provide the IP solution inmate calling.