that time not sure mah.... tot wah, wifi leh, free leh
roaming calls are the 2nd most profitable for telcos.
roaming data is the most profitable.
hence the expensive roaming plans from the MNOs.
if they can charge you $$$ for calls delivered over cheap wifi/internet, they will, and they will use the same rates as the expensive voice/3/4/5G rates.
give you an example (using sunset technology, but the same telco $ charging principles apply). fax over telephone lines is considered IDD, and hence expensive.
fax over IP, uses internet, and only uses telephone line for the final connection to destination machine (*), so it is local call. telcos hate this because they can't makan you.
*if destination is using fax server, it can be free. direct fax server to server over internet.
(MVNOs probably get lower bulk rates due to projected increase in use. lower rates but higher useage, still got profits)