The Internet Protocol (IP) is moving communications into a new era of all-IP integrated systems. Enterprises are demanding Voice-over-IP (VoIP) services to save costs as well as to achieve communication convergence. A VoIP gateway is a device that interfaces analog telephones with an IP-based network. The primary functions of a VoIP gateway is the translation of the analog telephone signals into a control signaling protocol as well as the voice packetization. VoIP is an emerging market where System-on-Chip (SOC) is developed. A SOC is a complete system on an integrated circuit (IC) that combines software running on embedded processor cores and hardware intellectual property. Nowadays, vendors offer a set of powerful tools and intellectual property blocks for designing complete and flexible System-on- Programmable-Chip (SOPC) in a very short time. This work presents the development of a low-cost and small VoIP gateway using the SOPC design methodology, and it also involves the study of different Power-over-Ethernet (PoE) solutions. This appliance is aimed to be used as a network test instrument within enterprise infrastructures. The Altera Cyclone II low-cost FPGA family and Si3210 ProSLIC are selected as the development platforms because this application is targeted to moderate volumes of productions, and also it is not able to afford risks and long design time. The design flow starts with the definition of a clear system model, followed by hardware and software co-design, and eventually prototyping and verification. This design actually exploits the synergism of hardware and software inside a single FPGA platform. The processes described in the system model are split in hardware and software. Thereafter, dedicated digital hardware blocks are implemented with VHDL to accelerate voice packetization, whereas a C/C++ program implements the main VoIP signaling protocol – Session Initiation Protocol (SIP). Altera SOPC Builder version 5.0 is employed to create the system architecture. The Nios II soft-processor core and set of on-chip peripherals are put together using a flexible interconnection bus structure – the Avalon Switch Fabric. Then, embedded software is designed to run over this architecture. It is built up and debugged using the Nios II integrated development environment (IDE). In addition, the program comprises of a group of tasks based on the popular MicroC/OS-II real time operating system (RTOS), which makes use of a lightweight TCP/IP stack (lwIP) for handling network applications. Finally, in order to verify the system functionality, the system is prototyped with the Altera Nios II Development Board that has a Cyclone II FPGA and the Si321xPPT-EVB. First, the designed architecture is configured into the FPGA, and later the embedded software is also downloaded on the evaluation board. Additionally, a ProSLIC device is employed to interface the SOPC inside the FPGA with an analog telephone. This ProSLIC device contains some other functions, and it is provided by Silicon Labs with a handy evaluation board. The prototype is tested with a SIP-based telephone and a SIP proxy server within the Ericsson Enterprise network. The SIP signaling and the transmission of voice packets is verified and proven correct.