Eg. WhatsApp and Viber works on only within the app but Simba and MR are able to call to any other Telco users. So this 3rd party VoIP operators work like former or latter? If it works like latter, which VoIP operator and is it free? It also means the traffic light Telcos can call with 4G devices without VoLTE capability!
Whatsapp/VIber etc do not use a phone number for calling, it is based on the app's account.
You can't call an regular phone number or vise versa using whatsapp/viber.
In a way you can say it is similar to Simba's voice app, but without the tie to the Telco's subscription.
It is phone number based operation, closer to commercial PABX, without ties to any specific telco.
Common implementations uses a voice app (can be specific or generic) that connects to a VoIP PABX setup (often paid) and typically you get an VoIP phone number as part of the service. This number is reachable using regular phone system, and only requires a data connection (including wifi) to operate. LIke what Logan mention the key advantages is that it works worldwide (unless specifically blocked) and you are not dependent on any specific telco or datacom technology. This is particularly an advantage if your telco blocks wifi-calling overseas.
Very common in North America. Examples include VoIP.ms, Fongo etc.
I am not familiar with SG examples.