MJ expects you to remain connected all the time. As you may remember from reading other postings, MJ makes a small amount of money on every call you receive. If you are not connected you cannot receive any calls and MJ cannot earn this incremental money.
Doesn't MJ make some money from received calls even if there is no answer and voice mail kicks in?? For that matter, don't they make money if you just dial into your voicemail (from something other than Ymax)?? This all can be done with or without MJ/ATA plugged in.
YMAX, the real phone company, receives a small amount of money through revenue settlement for every successful inbound call. This naturally includes calls that are successfully completed to voice mail.