What is happening is your system is designed to ""answer"" the phone when someone calls, hangs up, waits 10 seconds ( but not more than 40 seconds) and calls back. You system will answer with ""System on.""
This is Remote Phone operation that allows you to arm, disarm and even get system status from a remote location. In your situation, someone may have called you and hung up and then re called you. The Simon answers at the same time you do and that is why you and the other party can hear it.
A fix or solution to this issue is to enter installer programming then adjust Option 27 RING/HANG/RING to ""4"" or ""6"". To do this, you will need to open the cover to your system, enter your installer code, then [ADD] followed by [OPTION], . You will then enter  followed by [DONE].Hope this helps.