A subscriber with User Transfer, can transfer an established call to another line by performing a switchhook flash,  receiving a second dial tone, dialing the third party, and performing a disconnect at any time following completion of dialing.

Establish a call quickly press receiver button or flash key once and release it.

Wait for three short tones. Dial the number of the third person.

Wait for an answer. Then quickly press receiver button or flash key once and release it to bring the first person back for the three-way call.

If the third person doesn't answer or if the line is busy, press receiver button or flash key twice to stop the ringing or busy signal. This will connect you to the first person.

To drop out of the call just hang up after you have the three way call established and the other two people will remain connected.