My sister's Samsung Galaxy s6 Edge just started having this problem of writing not connected to network when you dial a number to call.
She bought it brand new in Philippines as at last year when she was schooling there, it was working very fine here until I came back and she told mahout the problem. I didn't want to stress myself finding the solution so I advised her to go to Samsung repair shop in fine brothers but she came back and from her story, the attendants there seemed more confused than her so I decided to help.
I've spent the whole night searching the web for solutions to similar scenario but all my effort in that respect were futile, I first thought it was imei problem but the imei is normal, I checked the download mode and recovery to check if it has been tampered with but the the Knox count is still null. I went for the last resort which is to flash it with a new firmware, it came previously with Android 6( marshmallow) so I upgraded it to Android 7( Nougat), I factory reseted it after flashing it with Odin but the problem still persists.
Am asking and appealing if someone have encountered such problem of not connected to network and how it was solved because right now am starting to feel this is a hardware problem. The phone uses a nano SIM and below is some screenshots of the phone.

Is the phone Network Carrier Factory Unlcocked ?

Is the phone network unlocked? Check the baseband if it's not damaged

How do I find out the baseband is alright?

The files before flashing.

Your IMEI looks suspicious. I suspect this might be a test unit, not one meant for retail. I suggest you tell your sister to return it to wherever she bought it.

Got one same s6 edge(only d panel now) with same problem which i've not been able to resolve. Previous attempts to fix shows its hardware related(sim slot). I've flashed, format, root, downgraded, upgraded like you, none helped. If u find a solution, please share

Seems its also model version related(my opinion). International unlocked version

dustmalik:
Your IMEI looks suspicious. I suspect this might be a test unit, not one meant for retail. I suggest you tell your sister to return it to wherever she bought it.

True, that IMEI looks really suspicious even at first glance.

IMEI checker returned this, not sure how true that is btw..

check on this thread to resolve your issue https://www.nairaland.com/3976749/repair-screens-samsung-galaxys-iphones

Baroba:
IMEI checker returned this, not sure how true that is btw..

Wow this calls for concern. Try another imei checker website and see what the result is

Or find a phone engineer who has Octopus box. If you find one, ask if they can re-write the imei. That should solve the problem if you can find an engineer who has Octopus box

OP quote replies in order to make us follow the thread

The phone is an unlocked international version, never been locked to any network. That's why it's showing blacklisted because it is not on clean imei server. Nothing is wrong with the imei since the imei is not null. Such imei are found on cdma and unlocked phones. Still waiting for solution

My 64gb s6 edge board with same no network problem is available for sale. Rooted with custom rom. For use as ipod or parts. Contact me

s33d:
The phone is an unlocked international version, never been locked to any network. That's why it's showing blacklisted because it is not on clean imei server. Nothing is wrong with the imei since the imei is not null. Such imei are found on cdma and unlocked phones. Still waiting for solution

Are you suggesting that it's a CDMA phone?

s33d:
My 64gb s6 edge board with same no network problem is available for sale. Rooted with custom rom. For use as ipod or parts. Contact me

Did you buy the phone like that?

emrain:


Did you buy the phone like that?

No. Started after some months usage

s33d:


No. Started after some months usage

i think ur sister's phone is country lock , Samsung is fond of doing that, but it can be unlocked

zamanii:


Are you suggesting that it's a CDMA phone?

No. Can't be.

ccrazy:
My sister's Samsung Galaxy s6 Edge just started having this problem of writing not connected to network when you dial a number to call.
She bought it brand new in Philippines as at last year when she was schooling there, it was working very fine here until I came back and she told mahout the problem. I didn't want to stress myself finding the solution so I advised her to go to Samsung repair shop in fine brothers but she came back and from her story, the attendants there seemed more confused than her so I decided to help.
I've spent the whole night searching the web for solutions to similar scenario but all my effort in that respect were futile, I first thought it was imei problem but the imei is normal, I checked the download mode and recovery to check if it has been tampered with but the the Knox count is still null. I went for the last resort which is to flash it with a new firmware, it came previously with Android 6( marshmallow) so I upgraded it to Android 7( Nougat), I factory reseted it after flashing it with Odin but the problem still persists.
Am asking and appealing if someone have encountered such problem of not connected to network and how it was solved because right now am starting to feel this is a hardware problem. The phone uses a nano SIM and below is some screenshots of the phone.

Give it to a hardware to engineer to run some diagnosis on it to see if its a hardware fault though i strongly doubt that the cause of the issue will be from the motherboard, if that doesn't work then try reinstalling the rom again or check to see if the phone is a sub brand like "t-mobile, verizon or others" and try to ask for a solution in their forums