When registering a DECT mobile part (MT) with the base station (BS), the VoIP user ID (user) is formed from the DECT user ID (IPUI) according to a mapping rule and is used for registering an SIP account (SIPA). In addition, the SIP password (pw(AC)) is formed from the DECT authentication code (AC). Roaming or handover of DECT mobile parts (MT) in DECT systems featuring a VoIP connection can be accomplished in a simple manner by adjusting the DECT user ID (IPUI) to the SIP user ID (user (IPUI')) and adjusting the authentication code (AC) to the password (pw(AC)). The DECT mobile parts (MT) can continue to be used without change even when the same are connected to IP-oriented networks (IN), while said DECT mobile parts (MT) can be marketed for a wider range of uses.