A previous randomized controlled trial demonstrated that phone and letter-based motivational interventions with high expenditure gamblers had significant short-term positive effects on gambling and use of responsible gambling tools. This post-trial follow-up examined outcomes in gambling expenditure over 12 months.Observational study following a three-arm randomized controlled trial.Customers of Norsk Tipping (NT) gambling platforms, Norway.1,003 statistical triplets from the top 0.5% of customers based upon annual expenditure, matched on sex, age, and net losses. Mean age 53.4 years, 19% were women, mean yearly loss for 2016 was 88,197 NoK. Interventions and comparator Feedback intervention by telephone, letter, or a no-contact control condition.Primary outcome measure was gambling theoretical loss, derived from the Norsk Tipping customer database. Secondary outcomes were responsible gambling customer actions and whether the participant was retained as a NT customer.Per protocol analyses of triplets who received the phone call or letter as randomly assigned (n=596) showed a positive and sustained effect over 12 months: the telephone group showed a 30% reduction in theoretic loss (d=.44) and the letter group 13% (d=.18), both outperforming the control group with a 7% reduction (d=.11). The phone condition was superior to both the letter and control conditions in per protocol (p<.001) and to control condition in intention to treat analyses (ITT) (p<.001). Individuals in the phone condition took more responsible gambling actions. The letter condition had better outcomes than the control in the ITT only (p<.001). Over 93% were still customers a year after the intervention.Personal contact with high expenditure gambling customers in Norway that provided individualized feedback on expenditures was associated with reduced theoretical losses and greater use of responsible gambling tools over a 12-month period, compared with no contact. Telephone intervention with customers had a larger impact than a mailed letter.