Can't receive Landline Calls on Nextel i56...

I have a fleet of Nextel i560s, but starting some time last week three of our phones (all the phones with a 303 area code (all of our other lines are 720 area code) no longer accept phone calls from Landlines, however cell to cell works just fine. We have done a Master Reset on all the phones, but the problem is still occuring and Nextel said everything is good on their end..