I want to enhance the security of our communications link by implementing 'dial in answer mode' (aka reverse mode) for the remote client and 'answer in originate mode' for homebase using the Rn command. This technique has not been successful.

'Dial in answer mode ' is not supported with the chipset in the MT5600SMI. Some vendors used the R character in the dial string to enable this on a per call basis. The SCM chipset will accept this character in a dial string but not act upon it.