While on an active call, you can now place the call on hold by clicking on the Pause icon from the In Progress section or the Pause icon from the active call-bar:

Once parked, you can resume any call by clicking the Resume button from the In Progress section or the the Resume button from the active call-bar:

Once you've parked a call and want to start a conference or warm transfer, you can type a name or number into the +New Conversation button in the top left section of the app or click to call from Salesforce, Gmail, or (almost) anywhere else. 

Once you click to call or enter a name/number, you will see a dial card below your in progress call. Click on the call button in that card to initiate the next call! 

Both calls will be listed on the left side of the app in the Conversation list. You can hover over either phone number to reveal the pause and resume buttons.

Once you've connected with the second participant, you can merge the calls together by clicking the Merge button in the conversation detail section.

Next, click the call on hold that you'd like to merge. 

Then you're all set!

While on an active call you can park that call by clicking on the Pause icon from the In Progress section.

Once you've parked your first call, click the back to conversations button.

Next, enter a phone number into the name/number box you will see a dial card below it. Click on the call button to initiate your second call.

Once you've connected to the second party, you will see both active calls at the bottom of the window. You can hover over either to expand the pause/resume buttons!

To combine both calls, click on the merge button in the top conversation detail section.

Click on the phone number for the call you'd like to merge in. 

Then you're all set!