Aug 23 (Reuters) - Facebook Inc (FB.O) is letting some users make voice and video calls within its main app on a trial basis, aiming to make it easier to place calls without opening its standalone Messenger app. The social media giant spun out Messenger from its main app years ago, meaning users would have to download a separate app in order to send messages and make calls. Facebook has been trying to tie together messaging across its suite of apps and first enabled it between Instagram and Messenger last September. The move enabled users of each service to find, message and hold video calls with contacts on the other without needing to download both apps. It plans to eventually integrate WhatsApp into the mix. However, a Facebook spokesperson said on Monday that for a full-featured messaging, audio and video call experience, people should continue using Messenger.