Is there a generic version of Apple devices for VoLTE services

Sorry no.

Bear in mind that you are not addressing Apple here. This is a user to user tech support forum and we, just user like you, are not aware of the requirements that Apple may possibly require to cellular operators/carriers.

Please contact Apple directly, using this link:--> https://www.apple.com/contact/

Regards

Giulio

I tried to do a search, but I could not find the document you are referring to.

Check out this list, to see if it's in between (and please let me know):--> https://support.apple.com/kb/index?page=search&q=VoLTE&product=&doctype=&currentPage=1&includeArchived=false&locale=en_US

Giulio

Take a look at this support article:--> About the LTE options on your iPhone - Apple Support

All iPhones model 6 or later support VoLTE.

Regards

Giulio

You are most welcome