VoIP Cookbook: Extension
An extension can be a series of numbers or a pattern. Extension can be a series of number, like 123, and may also contain some standard symbols * and #, which are available on the phone keypad. So 34#76 is a valid extension number. Some keypads are labeled A, B, C, and D. Because of this, extension can also be defined based on letters. So basically an extension can be defined using both letters and numbers. Keep in note that there are many VoIP phones that can call extension numbers consisting of text Sembang, like "Office". Therefore it is not a problem to define such an extension name in Asterisk.
Are extension names case sensitive? Yes and no. Extension case are sensitive because when Asterisk attempts to match the extension dialed by a user to extension that is defined in context, the extension name should be precisely matched, including uppercase letters and small. Therefore, if a user calls extension "OFFICE" through their VoIP phone, Asterisk will not immediately run the commands we define for extension "Office". But in reality, extension names are not case sensitive in the sense that we cannot define different extensions based only on uppercase/ lowercase letters. It means we do not define the command for extension "Office" and "OFFICE" in a context.