Mobile users have fingernails of different lengths. This paper measures the impact of fingernail length on the use of tactile mobile phones. We first conducted interviews with participants wearing long fingernails. They reported difficulties and non-satisfactory coping strategies to hold their phone securely and acquire targets accurately. We then conducted three experiments comparing different lengths of fingernails (0 mm, 5 mm, and 10 mm). Our results quantify the drop in comfort and efficiency. We measured the range of incidental pitch angle on the surface, the comfortable and useful area of the thumb, and the target acquisition efficiency. 10 mm fingernails consistently decrease by 57% the range of the finger pitch angle, by 36% the comfortable area of the thumb, and by 24% the throughput when acquiring targets. This paper contributes guidelines for future inclusive devices and techniques to also support users with long fingernails.