You must have two separate telephone lines, each with its own separate telephone number, in your home or office. The second line cannot merely be an extension line.
At least one of the two lines (Line 2) must be analog or DSL with filter. Line 1 can be analog, VOIP, or DSL with filter.
Digital telephone lines such as in a PBX system are not compatible with the CapTel 840 phone and may damage the telephone or telephone network. If you are installing the CapTel 840 in an office or residential facility environment, check with the telephone system administrator to ensure that an analog port is available. A digital subscriber line (DSL) may be used with an appropriate filter.