Lumia 640 and Lumia 640 XL are eligible to receive the Lumia Denim update. Here is a step by step guide to get your hands on it:
Lumia 640 owners with subscription to EE UK or T-Mobile US will be eligible for native Wi-Fi calling functionality post-update. They can also access VoLTE compatibility active on specific wireless carriers.
Also, on cards are audio call enhancement and add-ons to cellular connectivity, stability for network and national roaming. The latest software version for Lumia 640 is 02177.00000.15184.36xxx/8.10.15148.160, says Microsoft.
As for users of the Lumia 640 XL, they are also eligible to get hands on VoLTE support via selected operators besides native Wi-Fi calling support if subscribed to EE in the UK. The update also brings in enhancements to audio call quality, Wi-Fi, connectivity, roaming and network stability, again. Lumia 640 XL comes with the version 02177.00000.15184.36xxx/8.10.15148.160 for the update.
Simply select the menu in this order– Settings > Phone Update > Check for Updates to access the latest update. In case, you are yet to get the same, it just might be due to your region and location.