Publisher Summary Multiple dual-mode phones are on the market based on different operating systems. Linux, Symbian, the proprietary Research in Motion (RIM) Blackberry, Google Android, and Windows Mobile are the most popular. All the devices come with a “native” dialer that has been carrier certified, but these do not support any unbounded mobile communications (UMC) functionality and are strictly cellular service products. Other products may now offer a SIP softphone, but it is not integrated into the “native” dialer and is severely limited in functionality. With the existing crop of commercially available dual-mode phones, any UMC functionality has to be delivered as an adjunct application. Without direct collaboration with the handset manufacturer or OS provider (Microsoft or Nokia), supplanting the native dialer with a UMC solution is not without its problems. For both Microsoft Windows Mobile-based and Symbian-based devices, the native dialer functionality is built into the OS structure and cannot be treated as a user application. For that reason, any UMC application must be designed to work collaboratively with the native dialer. Even in the best scenarios, there are behaviors that are not optimum.