REFURB 1PT CH E1 T1/ISDN-PRI NTWK MOD The versatility of Cisco 2610-51XM 2691 3660 and 3700 series routers is demonstrated by their broad support of E1/T1 connectivity and Integrated Services Digital Network Primary Rate Interfaces (ISDN PRI). Customers continue to deploy E1 and T1 circuits for Wide Area Network (WAN) connectivity ISDN dial-backup and digital modem termination in the Power Branch Office Environment.