... up to 120 IP users, can be working under NAT for remote IP users by DDNSRecording, Voice mail,Music ... and DDNS, Can be working under NAT; based on Linux OS, GUI for management;The RT20 is an All ... -in-One converged communication system for small and mid-size offices, home offices and ... supports built-in IVR, calls records, and downloads for color ring back tones.Application Digram: RT20 Series IP ...