An extension is the number your SIP (internet) phone is assigned when it registers to (logs onto) the MaxoTel VoIP service. Each SIP device on MaxoTel has a unique 5 digit extension number, and you can call other MaxoTel extensions for free! (Including extensions on other people's MaxoTel accounts - so that you can ring your friends for free if they have an MaxoTel account). You can have more than one extension for example one at your house, and one at your business - you can make calls between them for free.
You can set the outgoing caller ID (CLIP) of each extension individually, or have it default to a value you select for all extensions on your account. You can also choose to divert your extension to a phone number or voicemail if it is busy or after a number of rings. Between all of the extensions on your account, the maximum number of simultaneous calls you can make or receive is governed by the number of lines that your account has.