[Galaxy A3, A5, A7, A8, A9 Q&A, Help & Troubleshoot] Bryan48765:
Set the network type or RAT or network mode to LTE (4G) only, then the phone should stay on the signal even when the signal is low for a second, around -120 rsrp (signal), specifically it should normally follow the parameters sent by the network: qRxLevMin (it is half the rsrp), qQualMin (sometimes, it is the signal quality), sNonIntraSearch, ThreshServingLow (both are ranges around qrxlevmin, one for entry threshservinghigh, one for exit). These parameters should nornally not be followed with LTE (4G) only. But only if the signal drops below the limit for a second. Else, it is a waste of resources, both for you (battery, etc), and the network, though the network can deprioritize cell edge (low signal) users.

To do so, there are extended network modes (for sim 1) in a menu that comes with Android, it is called 4636, Testing, Phone info, Device info, etc. There are apps that open 4636 without root, that offer: mode selections, lte only, anything cellular related that opens 4636.

Hold the phone on its top, not its bottom, for testing, the primary reception antenna should be at the bottom for A 2016, you can check this in *#0011#, it is "ant rsrp diff", it is positive when the primary reception antenna is better, negative when the diversity reception antenna is better, the diversity antenna should in most cases power on when the cellular is in use and start measurement, in contrast to the primary that's just switched on.

The signal can be measured in *#0011#, rssi is the signal strength of the frequency, the signal strength of intra freq (same frequency) neighbors (other cell towers etc) may differ slightly anyway because it's missing a decimal point; rsrp (good maybe at least -120, get it closer to -1) is approximately the signal strength of the cell, it is a few antennas in the long rectangular boxes on a cell tower, note that it is the measurement of the reference signal not the data that is being sent to your phone; rsrq(good maybe at least -15, best -1), sinr (good maybe at least 0, best 40), are signal quality. The "registered plmn" is the currently connected operator (mobile country code and mobile network code), if it changes (different compared to the sim), the signal is low! The "PCI" is the currently connected cell, note that there are only ~500 of these, so to identify the cell you can use both PCI and TAC (region), if the pci changes and the signal strength drops, well, the signal dropped and a reselection is made to an inferior cell, which can then drop data... in this case contact your mobile network operator but they might not make changes if it only affects a few users. You may try to lock band/etc to the good cell that sometimes drops in quality for a second but overall better than the neighbour cells.

Here's a short list of scenarios that can cause problems in connectivity, not all.

1: Low signal, plain old low signal. If the phone can't register VoLTE the bands that can be connected to may be restricted on some MNOs.

2. The antenna is closed and this causes low signal.

3. Low signal of the serving cell, the signal of an inferior neighbour cell matches (above parameters!), the phone changes to another antenna on a cell tower, quality drops and so does the connection. On another cell tower or there are 2 antennas on 1 cell tower, for example 1 facing the azimuth 180 degrees, another 270, the phone being in 215 degrees is unfortunate!

4. The antenna is open and receives interference (and reselections, sometimes good) from neighbour cells.

5. Good signal but it has been echoed a bit too much.

6. Reject causes and others, where the connection has been disconnected for some reason. Probably not in this case! It is viewable in *#9900# > Take cp log > copy to internal sdcard. It is in the /internal storage/log/LitmusLog. Scroll down to the reject cause section, those are error codes.

None of these codes/volte are working: The CSC is not applied correctly! Flash stock rom and open the stock recovery or copy /system/csc/...csc.../system/ to /system/etc/.

I do not recommend anything EFS/PIT, making a backup of efs is OK!