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Each VoIP user has an address or number through which the user is contacted. Apps released by VoIP services allow you to use, in most cases, the username or phone number you got when you registered with the service. There are third-party apps that are service-independent, allowing you to use them with any service. These use SIP addresses.This article relates only to 3rd party software applications (Apps) that work with SIP services that are universally compatible. AKA - SIP-Based VoIP Apps - Session Initiation Protocol. This clearly excludes closed proprietary systems such as Skype and Google Voice. To better understand the distinction read [[IP Voice Communication]].