How do I delete a call or message from the history

On iOS:
By sliding your finger on the call line from right to left, you will bring up the "Delete" button.
Unfortunately, it is currently not possible to delete a call history in bulk.
The same is valid for deleting an entire message conversation. 

On Android:
Leave your finger on the call line to bring up a pop-up allowing you to delete this call from your history. 
Unfortunately, it is currently not possible to delete a call history in bulk.
The same is valid for deleting an entire message conversation.