Steps to enable Vodafone VoLTE (if it's not enabled automatically):

So it looks like Vodafone VoLTE would only work if you send a written request for it . How to do it?

Short version:
Register your email ID. Send a request to Vodafone customer care from that email ID, along with your phone number, model & IMEI number.

Detailed version:
1. Install "My Vodafone" app, log in using OTP, register your email ID through profile settings.
2. Open phone's dialer, type *#06# to get IMEI number. Copy it.
3. Go to Vodafone website's "Contact Us" section (under Help & support). Choose your location (your network circle). You'll get Vodafone Customer Care e-mail ID from there.
4. Now send an email to that ID from your registered account, saying that you want to activate VoLTE service, your IMEI number & phone number is bluh bluh bluh.
5. You'll get an SMS & 3 emails in response.
6. Done. VoLTE would work now after rebooting.

[Note: This method should work on all devices, if the device is in Vodafone supported device list]…