How to enable 3G video calls on Android smartphone?
This article on the history and future of mobile video telephony from 2010 covers the challenges of mobile video telephony at that time.

The author lists out the then existing landscape and practical issues regarding video call adoption. Perhaps more importantly is to what the author sees in the future:


  What’s next?
  
  We do have mobile video telephony, with all of its benefits and faults. But where are we going with it? The next step will be a migration of the service from circuit switching to packet switching – to become all-IP. This will require two major changes:
  
  Migration from WCDMA/HSPA to LTE, where an all-IP network will be the norm and network capacities and bandwidths allocated for each phone will increase.
  Replacement of 3G-324M with a different standard that runs over an IP network. Probably as part of IMS (IP Muiltimedia Subsystem).
  While Apple just came out with their front facing camera and FaceTime service on the iPhone, it is still quite limited: it runs over WiFi, only between iPhone 4 devices and uses a protocol that Apple plans to open. For mobile video telephony to become a valid solution it needs to use an open standard, run everywhere and be interoperable across devices.
  
  When will that happen? At the very least 8 years from now it will require the creation of the necessary ecosystem of companies who care. The problem is that these companies are currently focused on providing the basics of the LTE infrastructure. This requires them to rethink their voice and SMS technologies in initiatives such as VoLTE (Voice over LTE).


So 


To get mobile video calling operators/carriers were mandating handsets support 3G-324M (a 3GPP specification), at least until the iPhone arrived.
Operators/carriers switched focus to providing the basics of LTE infrastructure. 
Apple hasn't opened up Facetime
Android itself has no requirement to support the 3G-324M, and Android Compatibility Definition Document shows no support beyond standard audio and video codecs so any support would have to come from manufacturers or carriers building it in themselves.
In the US, AT&amp;T and Verizon have announced end of 3G support, In Europe Vodafone has announced that 3G will be turned off throughout Europe between 2020 and 2021, and Deutsche Telekom plans to do the same in 2020 while in other regions/carriers it is still TBD.


So while older devices, like the Samsung mentioned had some type of 3G video calling support, should the associated carrier end 3G, support for 3G video calling will probably end as well, as the future lies with VoLTE which removed the circuit switch requirements.

If you are looking for older devices which does support 3G-324M,
from: https://whirlpool.net.au/wiki/android_video_calling


  The only modern smartphones known to implement 3G-324M video calling are the Samsung Galaxy range:
  
  
  Samsung Galaxy S4 +s5 +s6
  Samsung Galaxy S3
  Samsung Galaxy S2
  Samsung Galaxy Note
  Samsung Galaxy S I9000
  


Note that the most recent device that appears to have supported 3G calling on a carrier like Telstra AU is the Samsung Galaxy S5 whose last official update is Android 6.0.