Area codes 301, 240, and 227 serve the western half of Maryland. Area codes 410, 443, and 667 are telephone area codes serving the eastern half of the U.S. state of Maryland, including the Baltimore metropolitan area and the Eastern Shore. The 301 area code was one of the original area codes established in 1947, covering the entire state. However, due to the increasing demand for telephone lines, especially in the Washington suburbs, Maryland was divided into two area codes in 1991, with the eastern half receiving the new 410 code. To further address the growing demand, the 240 area code was introduced in 1997 as an overlay to 301. Later, in 2021, the 227 area code was introduced as an additional overlay for the region. Despite the introduction of new codes, 301 remains in use and is one of the few original area codes to have never been split.
Maryland has the second-highest share of adults who primarily use a wireless phone nationally at 21.9%. Data from 2023 shows that 65.6% of adults in Maryland rely solely on mobile phones, up from 44.2% in 2019—a gain of over 21 percentage points in four years. Maryland residents who primarily use a wireless phone make up 21.9%, while 6.7% rely on both mobile and landline. Those who mainly rely on landlines and those who only use landlines have declined to 2.7% and 1.8% respectively. The share of adults lacking phone access stands at 1.3%. Maryland's large segment of those who prefer mobile but still keep a landline suggests many residents maintain a landline as backup while primarily relying on mobile.