Our service provider is Verizon, all lines from the same provider, 6 in total.
All four lines plugged into one of the gateways are in a hunt group. The other gateway has two lines that are plugged into it, but these are not in a hunt group.
It's hard to say if the firmware upgrade was when it started, because we updated everything to the latest firmware when we first installed the system (November). I did read elsewhere that people were able to fix this problem when they roll back though.
The error message we hear is 'Sorry, all lines are in use. Please try your call again later.'
Let me know if you need any other info.