Hello there! Welcome to the forum! 
You have a working fax machine attached to a phone line. The phone line is... say... 555 222-1313. So someone from another area code can send a fax to that number (1-555-222-1313), but if someone from the 555 area code tries to send one, do you or they get an error? Does it show to go through on their end?
Are there settings on your fax machine where you could have inadvertently blocked certain area codes?
You have a working fax machine attached to a phone line. The phone line is... say... 555 222-1313. So someone from another area code can send a fax to that number (1-555-222-1313), but if someone from the 555 area code tries to send one, do you or they get an error? Does it show to go through on their end?
Are there settings on your fax machine where you could have inadvertently blocked certain area codes?