Yes you will still receive all SMS whilst on VoWiFi.

110% positive. My VoWiFi is on all the time. So it will change from Kekal Selamat - Digi to Digi WiFi Calling whenever I’m connected to a WiFi automatically. It worked flawlessly when I was overseas too and this was tried and tested in multiple Asian and European countries.

Whenever you are successfully connected to a local WiFi and your phone mobile carrier network displayed as Digi WiFi Calling, incoming call, outgoing call and even SMS are all treated as usage within Malaysia, thus everything will count towards your local minutes/SMS quota. So if you’re on unlimited call plan, you do not have to pay anything extra. All you have to do is to make sure your mobile carrier network is on Digi WiFi Calling when you are making or answering the call overseas. Occasionally the network will drop and you’ll be connected to Digi’s roaming partner most likely due to unstable WiFi connection. But not to worry as the line will be automatically cut off when that happens. So you don’t have to worry about roaming fee bill-shock.

I had no problem receiving promotional or banks SMS from the 5 digits number.
Digi introduced VoWiFi in 2017 but disabled the use of VoWiFi during the early days. In early 2018, someone reported from Amsterdam that Digi VoWiFi worked there and he had used it to receive calls but was not charged a single cent. I tried it out myself in Kyoto and Osaka, Japan a few weeks after reading the post and it really worked. Since then I had used VoWiFi in Seoul, Hong Kong, Taipei, Taichung, Kaohsiung, Bangkok and multiple countries in Europe.