Is voLTE on iPhone 5S possible? The LTE chipset inside an iPhone 5S is voLTE capable. Could Apple enable it just by an OS update? Just like all other companies are doing?

Hello,

Sorry voLTE requires an iPhone 6 or later.

VoLTE is available only on iPhone 6 or later.

Heres where i got it(at the bottom):

Wireless carrier support and features for iPhone in the United States and Canada - Apple Support

Not possible according to

Make phone calls with Voice over LTE (VoLTE) - Apple Support

An iPhone 6 or later is required (see the footnotes)

There's likely some hardware limitation of the iPhone 5s that prevents it from being compatible. What those limitations are, we cannot speculate about. Only apple knows for certain.