Just connect up and work your connection out through trial and error. After each configuration, use your line tester to see if you've got it right. When the telephone receiver won't come apart, you should attempt to disassemble the body of the phone. Look for the point where wires enter the phone from the wall and try to connect your alligator clips to exposed portions of wire. Again you'll probably resort to a little trial and error.
Phones also work off low voltage.
While caution should always be exercised to protect you, your computer, and the phone, the low voltage certainly limits the risk of personal injury.