I remember seeing it somewhere that mentioned customers who are using multi SIM one line is not capable to use VoLTE apart from the MAIN SIM

If that's the case, it's highly likely you're being shitted upon. VoLTE on iPhones are no different than VoLTE on Android devices, just a matter of configuration from Maxis in their database that your phone(device)/ multi-SIM lines are whitelisted or not, just call them and ask if they can provide a solution or not?

If not, do what I do, lodge a report to MCMC straightaway (I'm using Hotlink Prepaid), I used their Terms and Conditions against them where it mentioned Prepaid Unlimited Plan users can enjoy VoLTE since March 2021 where I didn't get VoLTE, few days later, they called me and informed VoLTE has been enabled for me.

Customer loyalty means nothing nowadays,

1) We have choice (Shitty choices when it comes to Shitty network providers in Malaysia, none are superb, just "meh" and all around average service across)

2) We like cheap prices but quality service/ goods

3) Gotta do with ego, I myself am guilty

15 years, those Zerolution packages are just right at your fingertips if you wanna use them. But up to you though