Welcome to the Google Pixel Community forum.
Yes, the Pixel 4a is both VoLTE and WiFi calling capable, as are all Pixel phones. However, in order to function, those features must be provisioned by the phone's mobile carrier, e.g., if a mobile carrier does not provision Pixel 4a phones to use those features on their network, Pixel 4a phones will be unable to use them.
Some mobile carriers opt to provision these features only on phones they themselves sell, and/or on certain select brands rather than all brands. In addition, phone brands that are not sold in a particular country will be unlikely to be provisioned by carriers in that country to use features such as VoLTE and/or WiFi calling.
Best,
Lilybell2