You can use the call merge feature when you are in a call, receive a call from someone else, and want to join all three lines into a single conversation.

Merge Calls from Your Phone

1. While you are in a call with someone, a new call will come in. Press Answer. The original call will be placed on hold. 

2. Speak to the new caller. Press Join to merge their call with the original call.

Note: You can add additional participants to the call. To do so, tap Hold to place the three-way call on hold. Tap New Call and dial the number of the additional participant, or select their name from your Recent or Favorites list. When they answer the call, speak to them to let them know you are adding them to a call, then press Join.