Done some digging and research of my own and from reading others complaining of poor signal strength due to zero or 1 bars on their Sony or other Android phones. This may be of some help and use to others.
Sony and other Android phones reduce and save energy to the mobiles antenna (reception) by default on android 7 or 8 o/s (2016+). Soon as it detects an incoming or outgoing call it provides the power to the (phone) & antenna and the signal (reception) jumps to 4/5 signal bars even from zero.
So don't worry if you have 0 or 1 signal bars showing, its normal ! and reserving battery power only.
You can verify this by checking below yourself.
To note the reception bars icon is an bad indicator, the more accurate correct reading is here:
(android 8.0 should be similar for other versions)
Settings>System>About phone>Status>Sim status
Then check under Signal Strength, it should read a number roughly -70 to -120 dBm 30 asu, should be a minus followed by a number though. The lower the number the better the signal, mine sits around -100 or -115 inside my flat. But soon as I make a call or receive a call it improves to -85 or -95, which is I believe 4-5+ more better or "on".
Also if you are having poor signal, check the case (lid or gel or plastic case) or screen protectors on front is not blocking the attenna, you can remove it and retry, just be aware the screen protector may need to be replaced due to glue wearing off.
My actual fault did not lie with the phone or software or hardware it was a faulty sim, placed a new sim with new number and my signal break up issue was fixed.
A very weird problem but glad its resolved, hopefully helps others out.
