Developing a SIP-based telephony service requires a programmer to have expertise in telephony rules and constraints, the SIP protocol, distributed systems, and a SIP API, which is often large and complex. These requirements make the development of telephony software an overwhelming challenge. To overcome this challenge, various programming languages have been proposed to develop telephony services. Nevertheless, none of these languages as yet has a formal semantics. Therefore, the reference implementation, which may not be available, becomes the only source of information for the programmer to understand the subtleties of the language. Furthermore, this situation makes it difficult for third-party developers to port the language to another runtime system or to provide another implementation of the runtime system. This paper presents a semantics-based stepwise approach for designing and developing a scripting language dedicated to the development of telephony services. This approach enables critical properties of services to be guaranteed and captures expertise on the operational behavior of a service. We have applied this approach to developing the Session Processing Language (SPL) [3] dedicated to SIP-based service creation. A variety of services have been written in SPL for our university department.