Right now it is just on windows - there is a curses based version that runs on Unix but that hardly counts. The SMIME stuff still needs work for key ring management stuff and the TLS has not really been tested with
You can download the windows install program SIPimp.msi
If this does not work for you and claims you need a more recent installer then download all the files in SIPimpSetup.zip then unzip and run setup
My configuration looks like
184.108.40.206 is the public address of my NAT and I have forwarded port 5002 on it to the private address of my computer I run this on. I also needed to set my local port to be 5002. If I was not behind a NAT I would leave the contact blank.
If you want to try the TLS or S/MIME stuff - it is currently complicated and I don't really recommend it unless you like playing with sort of stuff. You need to install your certificates - this is lame - a better way is needed. Create a base directory to put them in - the default is C:\certs. Put your public cert in id.pem, your private key in id_key.pem. Put a root certificates you use for checking things in root.pem. Now it gets worse. Create sub directory called public_keys and in this directory store the public certificates of others. If the others AOR was sip:email@example.com, put their public certificate, in PEM format, in a file called firstname.lastname@example.org. These are only laded when the program starts. If you receive a new certificate from someone else, it is NOT written into this directory. Don't forget to set the password to decrypt your private key in the configuration dialog.
There are many known bugs.