Purpose: Researchers have used unscheduled telephone calls for many years during clinical trials to measure adult stuttering severity before and after treatment. Because variability is a hallmark of stuttering severity with adults, it is questionable whether an unscheduled telephone call is truly representative of their everyday speech. Method: The authors studied the speech of 9 men and 1 woman for a 12-hr day during different speaking activities. On that day and 1 week prior to that day, participants received an unscheduled 10-min telephone call from a person unknown to them. The authors compared the percent syllables stuttered (%SS) for the unscheduled telephone call on the day to the %SS of the unscheduled telephone call 1 week prior to the day and to the %SS during the entire day. Results: No significant differences were found, and all confidence intervals with "t" tests included 0. The concordance correlation test also showed a strong positive correlation between %SS scores for the entire day and for the unscheduled 10-min telephone call. Conclusion: The authors conclude that there is no reason to doubt that 10-min unscheduled telephone calls are a representative speech sample for %SS during clinical trials of stuttering treatments.