← Area code 603
603-503 phone numbers
Numbers 603-503-0000 to 603-503-9999 are assigned to Mcimetro Access Transmission Services LLC in Lancaster, NH. 4 of its ten thousand-number blocks are assigned to other carriers (number pooling).
Not assigned to any carrier: 603-503-1xxx, 603-503-3xxx, 603-503-6xxx, 603-503-7xxx, 603-503-8xxx — calls from these use spoofed caller IDs.
Source: North American Numbering Plan Administrator, Nov 3, 2026. Numbers can move between carriers.