In much of the world it is possible to tell whether a phone call originates from a landline or cellphone simply by looking at the phone number. In the United States, however, one must do a little research to answer this question, typically by using some version of a reverse phone lookup service online.
In most countries in the world, mobile phones are either given designed mobile phone codes to allow them to be easily identifiable, or are assigned a specific range of numbers for the same purpose. The largest exception to this system occurs within the countries that are part of the North America Numbering Plan, specifically the United States, Canada and some Caribbean countries. In these countries, mobile phones are assigned to a geographic area code that may end up being very far away from where the phone is principally used. Although the Federal Communications Commission is considering a proposal to allow VoIP calls to have their own area code, there is no viable proposal yet to give mobile phones an overlay area code.
What this means is that people who want to know whether a given phone number belongs to a landline or mobile phone must use a secondary service online to determine the source of a phone number. Phone Validator is one of the services that provides this information, and many others are found by entering "reverse phone lookup" in a search engine. While many of these services require a fee to deliver information regarding who a number belongs to, most disclose whether a number is a landline or mobile phone for free.