In Cambodia, HIV is prevalent in several high-risk groups including among female entertainment workers (FEWs) who work at entertainment venues such as karaoke bars and massage parlors and may engage in direct or indirect sex work. Reaching FEWs with sexual and reproductive health services such as testing and treatment for HIV and sexually transmitted infections (STIs) and contraception has been difficult because they are hidden and stigmatized. Mobile phone-based interventions may prove to be an effective way to reach this population and connect them with the existing services. This article describes study design and implementation of a two-arm randomized controlled trial of a mobile health intervention (the Mobile Link) aiming to improve sexual and reproductive health outcomes among FEWs in Cambodia. A two-armed randomized controlled trial (RCT) will be used to determine the effectiveness of a mobile phone-based text/voice messaging intervention. The intervention will be developed through a participatory process; 50 FEWs will work alongside researchers in focus groups to modify and tailor behavior change theory-based text and voice messages. Then, 600 FEWs will be recruited and randomly assigned into one of two arms: (1) a control group and (2) a mobile phone message group (either text messages or voice messages, delivery method chosen by participant). The primary outcome measures include HIV testing, condom use, STI testing and treatment and contraceptive use. If the Mobile Link trial is successful, an increase in condom use, screening and treatment for HIV and STI and contraception use is expected. These outcomes would lead to a reduction in the prevalence of HIV, STIs and unintended pregnancies. This trial is unique in a number of ways. First, the option of participation mode is offered to allow participants to choose the message medium that best links them to services. Second, this is the first RCT of a mobile phone-based behavior change intervention using SMS/VMs to support linkage to sexual and reproductive health services in Cambodia. Third, we are working with is a hidden, hard-to-reach and dynamic population with which traditional methods of outreach have not been fully successful.