Who I have the same questioned this topic

First when setting them up, caller received message that said no mail box was set up.  Set up mailbox and now callers just go to mail box.  It doesn't matter if they are on our wireless network or out and about.  I can't get through but they can get through to me.  This has been like this for months.  Finally frustrated enough to look for a solution.  iPhone 6 was an upgrade from a Motorola RAZR and the 6 Plus was from a Verizon home phone.  Either way nether works.  You would think this would just work like my Samsung Galaxy 4 did, but it appears some setting or something on Apple phones have to be set or avtivated.  I see this conversation from as far back as 2011 but no solution post for view as the conversations seem to drop off.  I also set the Do Not Disturb feature to allow calls from everyone since it defaulted to favorites. There certainly has to be a cure or no one would be using these phones, however, it is shocking the steps to solve this problem isn't in the notes that come with the activation steps.