NettiTieto Oy wanted to expand their call system by adding VoIP functionalities alongside with their existing GSM system. The purpose of this thesis was to find a suitable SIP library for making and receiving VoIP calls. The pur-pose was also to make one successful VoIP call from the call system to a smartphone.

The thesis was done in three stages. The purpose of the first stage was to find different libraries and software de-velopment kits, to compare them to pick the most suitable one. In the second stage, the purpose was to develop an application that uses the selected library. The application was coded with the C# programming language. In the third stage, all desired VoIP functionalities were integrated from the application to the call system.

The final result was a good set of VoIP functionalities for the call system. The goals of the thesis were achieved.