You HAVE to use a MSLC 2 for proper In-Band Integration. In the past, I have used Vodavi's Analog Dispatch . Integrated real well. Of course you can use Starmail, if you can find one.

I have seen Aleen as a possible system. I have seen it on a DS616, so it should also work with the 56/120. You won't have all the same display features, but it works.