Speed selector

Hi allHaving a great time with the PMDG 73* range. However, today I had a problem which I am unsure if it's a software or user error. I began a descent from FL310. I first select about 1500 VS to allow the engines to come back midway, then after a few minutes I will use the FLCH mode. This time around I had selected VS mode then the phone rang (its at this point pilot error could be identified). The cruise speed was 0.78mach, and as I was busy this was the selected speed all the way down to 4000alt. After the phone call I stepped back into the flight deck and tried to dial the speed back, but was unable to either change the Mach/knots display or change the MCP speed. My only option was to disconnect the A/T.(PS aware of the transition level alert)Kind regardsPaul Edwards