CPHS is an extension to the GSM specification that adds several convenience and usability enhancements to the way GSM phones and SIM cards operate.
Examples of CPHS enhancements include single-key access to a standard voicemail number stored on the SIM, and a message-waiting indicator to alert users to new voicemail messages. CPHS also provides carriers with greater control and flexibility over the carrier name that is displayed on the phone.
CPHS also includes the ability to control certain network-based features from the phone interface, including call forwarding, call waiting, and caller-ID.
CPHS is not an official part of the GSM specification. It was developed by the PCN Association rather than the GSM Association. Nonetheless, CPHS has been popular among carriers. Many European and American GSM carriers require it, and so newer GSM phones from most major manufacturers include CPHS functionality.