In my area (Thirupporur OMR), BSNL's 700 MHz network performs well indoors and outdoors. Indoors, I get 3 bars of coverage with speeds around 7 Mbps, while outdoors, the coverage is full with speeds of 30 Mbps. However, I've noticed that BSNL VoLTE does not work on Samsung devices. I tested this on the S22 Ultra, S23, and M35 models, trying all possible troubleshooting steps, including sending a VoLTE activation message, updating the phone, and even visiting the BSNL office, but nothing worked. Interestingly, VoLTE works fine on Moto and Realme devices. I mainly use BSNL as a secondary SIM for data, while my wife and mother use it as their primary SIM for both calls and occasional outdoor data use, though they mostly connect via WiFi. Another issue worth mentioning is that while traveling on a bus or cab, the handover between towers isn’t smooth, which drains the phone battery quickly.
Ok do me a small favour. Download Google dialer app
Run the app make it a default app
After that just type in the dial
*#*#4636#*#* a window will open Go in phone information
After that go down where it is written this.
Set preferred network type search lte only. Select it do a ping test with mobile data on it should show pass. Come out of the app by closing it now dial any number and check if it is working in lte only now
Run the app make it a default app
After that just type in the dial
*#*#4636#*#* a window will open Go in phone information
After that go down where it is written this.
Set preferred network type search lte only. Select it do a ping test with mobile data on it should show pass. Come out of the app by closing it now dial any number and check if it is working in lte only now
Here it's 
Samsung Galaxy Smartphone: How to optimise network connectivity
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