You can't use Jio 4G Sim in 3g phone until 3G Network integration with RCom or BSNL takes place but you can still use Jio HD Voice/Video calls & faster 4G Internet without inserting Jio 4G Sim into 3G mobile.
3G mobile won't support Jio 4G sim due to hardware incapability but HD Voice/Video calls is possible if device includes WiFi & Front Camera. Jio is true 4g network which means no support for 2G or 3G but unofficial sources reported 2G & 3G fallback service to enter into Jio 4G network in future.
3G mobile won't support Jio 4G because VoLTE feature is missing, which helps to make HD Voice & Video calls on Jio 4G Network.
4G phones without VoLTE can use Jio 4G sim for data service. HD voice calls can be made after installing JioJoin – An app officially developed by Jio for enabling HD calls on Non-VoLTE phones.
Same is not applied to 3G mobiles – No LTE based hardware – No Jio 4G Sim !
But, 3G mobile users are not completely barred from Jio 4G service, 3G phone users can also enjoy complete digital services offered by Jio.
How to use Jio 4G Sim in 3G Phone
Already confirmed that Sim will not work until 3G network integration takes place.
Non-VoLTE 4G mobile users can use Sim only for data & JioJoin app for Calls/SMS while 3G mobile users can use JioFi for 4G data & Jiojoin app for HD Calls/SMS.
3G mobile users have no choices over Jio 4G except buying JioFi for when users need to make HD Calls/SMS and use Hostpot for 4G internet.
JioJoin app will work in both VoLTE & Non-VoLTE 4G mobile as well as 3G mobiles – Conditions is JioJoin detects sim already inserted in 4G mobile or, 3G mobile connected to JioFi over WiFi.
JioJoin in 3G mobile will only work when device remain connected to JioFi WiFi network.
JioFi reviewed as most easy-to-use 4G MiFi module developed by LYF. Anyone can easily setup JioFi to access Jio 4G internet on the go.
3G mobile users can easily buy LYF Flame series 4G mobile instead of JioFi to experience Jio 4G network but buying LYf Flame series mobile over JioFi would become a bad decision at the end, you can compare LYF Flame vs JioFi to get complete idea why JioFi is a better choice over LYF Flame series mobiles.
How to make Jio HD Calls and use 4G Internet in 3G Phone
All you need is to buy JioFi @Rs 2899 from Jio Store. Now follow JioFi complete setup & FAQ page to activate 4G data & Voice/Video calls.
After completing initial setup, you need to connect 3G mobile to JioFi over WiFi and install JioJoin app to make HD calls & SMS.
Carrying JioFi is very easy as device weighs very light & gives up to 8 hours of battery backup on moderate use of Data, Calls & SMS. Incoming & outgoing calls from Jio sim on 3G mobile is possible as long as 3G mobile remains connected to JioFi over WiFi.
You may face slow-speed in JioFi – It's not an issue to worry : Be sure to lock JioFi to Band 40 for best 4G speed and Band 3 or 5 for better network coverage.