Dial 7 digits in an area requiring 10 digits
Try the Dialer One app. It is a pretty good 'Phone' replacement app. Under 'Advanced Settings' there is an option to specify 'Phone prefix'. This prefix will be added when you long click the call button. I tested by setting the prefix to '402'. I typed '8675309' and then long pressed the call button. It prefixed '402' and the number became '4028675309'. You can press the call button again to call the number.