In the United States, little is known about interventions that rely on mobile phones and/or text messaging to improve engagement in HIV care for vulnerable populations. Domestic studies using these technologies as part of the National Institute on Drug Abuse "Seek, Test, Treat, Retain" research initiative were queried regarding intervention components, implementation issues, participant characteristics, and descriptive statistics of mobile phone service delivery. Across five studies with 1,135 predominantly male, minority participants, implementation challenges occurred in three categories: (1) service interruptions; (2) billing/overage issues, and; (3) the participant user experience. Response rules for automated text messages frequently frustrated participants. The inability to reload minutes/texting capacity remotely was a significant barrier to intervention delivery. No study encountered confidentiality breaches. Service interruption was common, even if studies provided mobile phones and plans. Future studies should attend to the type of mobile phone and service, the participant user experience, and human subjects concerns.