You do not have to operate OTRS and/or OTRS::ITSM yourself. The products can be rented via the so called "ASP" (application service provisioning) resp. "SaaS" (software as a service) model from specialized companies.
The customers (software users) are permitted internet access to exclusively rented OTRS systems and, where required, functional application support (see section above) at a fixed monthly price and can employ the application in their business to the contracted extent. No additional license fees apply as only open-source products are used.
The application service provider operates IT infrastructure, systems and software ITIL compliantly and guarantees service quality according to the agreed service levels. The provider maintains the application system, (e.g. patches, backup, monitoring), and supports the customer with incidents and/or service requests such as consultation requests, software extensions, or configuration requests.