Background Research is often undertaken using patient-reported outcomes from questionnaires. Achieving a high response rate demands expensive and time-consuming methods like telephone reminders. However, it is unknown whether telephone reminders change outcome estimates or only affect the response rate in research of populations with low back pain (LBP). The aim is to compare baseline characteristics and the change in outcome between patients responding before and after receiving a telephone reminder. Methods This is an ancillary analysis of data from a prospective cohort study employing questionnaires from 812 adults with LBP lasting more than 3 months. Patients not responding to the 52-week questionnaire were sent reminder emails after two and 3 weeks and delivered postal reminders after 4 weeks. Patients still not responding were contacted by telephone, with a maximum of two attempts. Patients were categorised into three groups: 1) patients responding before a telephone reminder was performed; 2) patients responding after the telephone reminder and 3) patients not responding at all. A positive outcome was defined as a 30% improvement on the Roland Morris Disability Questionnaire after 52 weeks. Results A total of 695 patients (85.2%) responded. Of these, 643 patients were classified in Group 1 and 52 patients were classified in Group 2. One hundred seventeen were classified in Group 3. No differences in outcome or baseline characteristics was found. In Group 1, 41.3% had a positive outcome, and in Group 2 48.9% had a positive outcome (P = 0.297). In group 3, non-respondents were younger, more often unemployed, more often smokers, more often reported co-morbidity, and reported higher depression scores than respondents. Conclusions Using a telephone reminder had no consequence on outcome estimates nor were there any differences in baseline characteristics between patients who responded before or after the telephone reminder. Trial registration The initial trial was registered in Clinicaltrials.gov (NCT03058315).