Device not falling back to 3G from 4G

[Android Central Question]

Hi,
I have a Lenovo vibe k6 note stock rom on Android 7.0.
Calls are working perfectly in Jio 4G (volte).
Recently I switched to Vodafone 4G (no volte).
When I try to place calls, the device is not falling back to 3G. I am unable to switch the network to 3G or 2G manually also. However 2G works when I change my primary data Sim.
I tried swapping the sim slots, toggling enhanced 4G services, restoring the device software, changing sim cards, replacing my sim card, the same problem persists.
I am using this sim with Lumia 640 XL LTE now. This issue is not present with other phones.
Any help would be appreciated.

 

[theflyingrahul]

Not if the carrier is using 2G for voice. Data is still on 4G. (It can't be on 3G.)

The carrier uses both 2G and 3G for voice. When I tested on other phones, the network automatically falls back to 3G from 4G when I try to place calls.

 

[theflyingrahul]

You're talking about Vodafone India, correct? What's the exact model number of the phone?

The exact model is K53a48 Lenovo VIBE K6 Note.
Yes the carrier is Vodafone India.

 

[anon(10614692)]

You do realise you cannot have both sims on 4G?