SIP user agent simulator

[Zhen . Yang]

Hi,

Does anybody know where I can get a piece of software that simulates the SIP
User Agent's (call control part) behavior? I need it just for testing. I
found a few, but they all come together with the protocol stack. I don't
need a stack at all. 

My department is planning to buy a SIP phone, there are some products
available. Does anybody have experience of using them? How about the
performance? Any suggestions of which one we should buy?

Thanks in advance.

Zhen