Phone not making landline calls

Hello, My new iphone 6 won't connect to local landline numbers. I can make and receive mobile calls but if trying a local landline it says the number I am calling is disconnected. I can call landline's that have a different area code to myself and but can only call local landline numbers if I put the area code in first. I have updated the software to ios 11.4.1, tried turning airplane mode on and off, checked diversions etc and removed sim. This is very frustrating as no local numbers in contacts will work unless manually dialed from keypad with an area code added. Please help !! Cheers.