This is very simple and basic setup for soft IP phone (software-based phone installed on PC);
You need 3725 IOS, MS Loopback, and Cisco IP Communicator (it might work with other software-based phones)
Assign Cisco IP Communicator phone to MS Loopback and set tftp server to IP of router interface connected to MS Loopback;
In my case: IP of Fa0/0 is 172.16.0.1 and IP of MS loopback is 172.16.0.10 (no default gateway) all in 255.255.255.0
Try it and let me know if you have problems;
Make sure your router has at least 256 MB RAM !