[help]developing sip softphone

hello all
I am having my project,according to which I have to develop a sip compatible softphone. I am not having any idea of development whatsover.
can u guys help me out?? how can i develop sip softphone??

To do that from scratch is several man years of work. Go back to your tutor and ask them which bits you must actually develop and for which bits you can use pre-existing libraries. Also ask what licensing terms are acceptable for any libraries you use.

Thanks david for your reply.
I think I can use any pre existing libraries for the purpose. As i said earlier I want to develop only sip softphone.
or u can say that a tool/software which can implement sip protocol. I dont care about how libraies/api implements sip protocol. Libraries can take care of that.
As this is college project,and it is for educational purpose only. no commercial use whatsoever.so I dont think licensing terms will apply for me. Also,as sip is open source protocol then I think enough open source libraries must be existent.