Can't Get MagicJack to Receive Calls

I recently reactivated a MagicJack Plus 2014. 

When plugged into my router, I can make outgoing calls but not receive incoming.  Figuring maybe I am just confused about what ports to forward, I unplugged my router from the cable modem and tried plugging the MJ in directly.  (I first plugged in a laptop, rebooted the modem, and verified a good connection on the laptop.  Proof of concept.)  Upon direct connecting I restarted the modem, and the MJ.  Again, I could dial out but not receive incoming calls.  To the caller, it rings for a time then goes to the unavailable message.

I am using my own Arris cable modem with Comcast internet.

I took the MJ device to a business which also has Comcast internet.  Upon making a direct connection to the cable modem, the MJ could dial out and now also ring for incoming calls.  I didn't even need to reboot the modem.

The only differences are that residential service vs. business and the Arris cable modem at home vs. a Comcast modem at the business. 

This sort of implies that Comcast blocks ports used by MJ on residential service but I see others have it working so I don't know.

One other factoid is that the Arris modem (modem only; I have a separate router but the router isn't a factor when direct connecting) is a model with telephony capability which I am not using.  I presume that is a completely separate thing just as cable tv service is a separate thing but I do not know.  I suppose if one had Comcast phone service that it is grafted onto internet with some ports going to it.  I don't think that is the case but that aside I don't know why it would work in one place but not the other unless Comcast is blocking some ports.