Digital phone systems are designed with a proprietary bus structure for adding features and capabilities. Boards are added to the cabinets for analog, digital, or IP phones. Features such as music on hold, VoIP integration, and alarm systems can be supported with modular add-on boards. Today, most digital systems, even if they use proprietary hardware or protocol, offer an IP interface on the controller. The IP interface might allow unified messaging features such as voicemail delivery to email, fax delivery to email, voicemail transcription to SMS, click to dial, and a desktop client. These systems are considered "hybrid PBXs" because they use a combination of proprietary digital hardware and standards-based IP networking. A fully modern digital PBX is 100% IP and software-based.