To begin a Voice call:

   1. Click the Hardware menu button.
   2. Click the Voice button.
  
You can also get to the "Voice" button by long-tapping one of your contacts.

Please understand that "Voice" and "Video" buttons will be grayed out if the features are unavailable in any given conversation, which can happen if you are conversing with a user who is using a version of the software in which the feature is not supported. Voice calling is supported on all Androids, iPhones, PCs, and Macs that support the Messenger application.