The study presented explores potential usability gaps when users switch from a familiar to an unfamiliar mobile phone interface. An experiment was performed where nine users familiar with Sony-Ericsson T630 and nine familiar with Nokia 7250 performed tasks on both phones. On average, test subjects spent more time to finish tasks with an unfamiliar phone than with a familiar one. For most tasks, there was no significant difference in completion time between the first-time Nokia users and the first-time Sony-Ericsson users. Only adding a contact to the address book was faster for new Nokia users than for new Sony-Ericsson users. In terms of both completion time and subjective ranking of difficulty, first-time Nokia users found this phone easier to use than first-time Sony-Ericsson users. Potential explanations for these observations are presented and discussed.