Partially it is Simba fault, but also partially the handset fault.
Because somehow Simba is not recognised when your handset perform a carrier check. That is why VOLTE don't work. For the 3 other Telco (and all other MVNO leasing the network from the 3 Telco), the carrier check will have no problem. As such, this is why all other Telco VOLTE works automatically. For Simba, you have to disable VOLTE carrier check in your phone.
Google different phone have different ways to do that. You have to Google it for your handset.
It is not recognised because the phone APN database does not have Simba in the list.
The other Telcos have been selling phones, it is easy for them to request the manufacturers to include them in the APN list.
