If the phone is less than 14 old, take it straight back to the shop you purchased it from.
They will look into the probs n if necessary swap the handset, with no issues....
DO IT NOW
I had my S8 for 6 mos. and was unable to use bixiy voice. The message that I kept receiving said the phone needed a software update. The phone itself kept saying that it was up to date and no updates were available. I did a smart switch update through a computer. Success! Well, almost. Upon powering back up I ended up with the "network not recognized".
It no longer recognizes my sim card. I placed the same sim card in my old Apple phone and had service again. I have sent this phone in for repair twice only to have it sent back with the same problem. I was informed by Samsung that it had to be repaired three times before a replacement could be considered. I'm sending it in again so that I can just get this issue resolved. I don't know why they can fix this when software updates were doing the same thing back when there were S3's.
I have been without the S8 for nearly a month now.
I am having the same problem and an really disgusted with this phone which was very expensive, and also with the lack of answers I have found. People who have not experienced the issue are not giving good advice. I wish I knew the answer. Have complained to Samsung and will take this back to the Sprint store
I took out sim card and wiped it off with rubbing alcohol. I haven't had an issue since. So in my case the contacts were just dirty. Hope this helps. Besides that, this really is a pretty good phone, once you disable bixby button.
This may not be a fix for everyone but this issue started happening to my husband's s8 back in October 2017. It got more frequent over time. I search all over the internet but none of the fixes worked until we came across a poster that said she changed her mobile network band to GSM. It was under an "AT&T" forum. My husband has an unlocked s8 that is used on AT&T's network. We did the following (and it has worked so far). You can find youtube videos on how to change the mobile network if you need visuals:
1. dial this number (like you were making a phone call): *#*#4636#*#*
2. You will then see a "secret" menu and can get to your mobile network band through selecting "phone info". You will need to scroll through slowly to find the line to select mobile band.
3. If you are on AT&T or T-Mobile, you need to select a GSM band. We chose "GSM auto (prl)".
4. If you are on Verizon or Sprint, select CDMA.
I don't know who runs on LTE, but that was what my husband's phone was set at and it was NOT working for At&T. It seems to be running even faster and he hasn't had the warning since but we changed it last night and it's not been proven 100% yet. I'll try to update with anything new. PS - I am no techie or expert so if I'm totally off on this, sorry.