The interim fix is to downgrade to the previous firmware for your phone. A link to that firmware was posted on one of the other conversations of this site. It is for the S9+, but firmware for the S9 is also available there. My concern for this solution is that the latest firmware will eventually automatically get re-installed again and I'm unsure how to stop that happening. It would appear that Samsung are not in any hurry to fix this problem.
You will need to find the correct firmware for your phone by referencing the model number - in my case,G965U1 for my US Unlocked S9+.
You will need to do a (free) registration with this site to be able to download the firmware.
You want the firmware dated 30th July and security patch level dated 2019-08-01 and with SH1 at the end of the firmware name. The firmware with SH7 at the end is the bad one.
I am having the same issues too after latest software update. I have a US Samsung S9 phone with a AU Telstra sim.
I still have my old US Verizon sim card in another phone. I put the US Verizon sim card in my US Samsung S9 phone and the issue completely went away. The phone went back to super zippy, responsive with no IMS error messages.
I then removed the US Verizon sim card and reinstalled my AU Telstra Sim card - issues returned immediately. Constant 'IMS Service keeps stopping' error messages with very sluggish phone response.
This issue is definetly Australia/NZ sim card related.
I've contacted Samsung Australia for support and received the following information:
"...However, the variant is USA. The reason why you are getting this error is because the over-the-air update you are receiving from Samsung USA is not installed properly. We suggest to contact Samsung USA so they can release a software update to patch the bug. They will help you manually download the update appropriate to the device. "
The problem now is that I cannot see a way to contact Samsung US support via chat (only a phone number is available). I have thus messaged them via fb (https://www.facebook.com/samsungsupport) and also used the DM link provided to@OzDeaDMeaT below.
I also started a thread on this issue (https://us.community.samsung.com/t5/Galaxy-S9/The-latest-update/m-p/821971#M26250), as I am experiencing the same problem. It seems there are a lot of people experiencing this. I have gone round in circles with Samsung Australia and Samsung US, who keep referring me to the other! I bough my unlocked phone in the US and have been using it in Australia. It was sold to me on the understanding that it was usable globally. It seems that there is still no solution offered, despite the length of time this has been going on. I am so disappointed with them.
Have had the same issue as everyone else here, in Australia with a US model S9, purchased through Kogan in April. Have contacted Samsung with not joy. Won't deal with it due to being a US model... Won't forward my fault request to the US support desk. Apparently they can't do that.
So, lodged formal complaint with Consumer Protection (Office of Fair Trading here in WA). Hopefully that might amount to something. Will see.
I also have the IMS keeps stopping problem after the latest update today. After reading that many have the same issue I rang Samsung Australia technical support. They have no answer to the problem except to take the phone back to Samsung in USA and hope there is a software update over there to fix it. This is unacceptable. It was suggested that the update was not meant for the USA purchased phone,I would like to know why it installed then. My phone is almost useless as it is sluggish and battery is draining quickly.
After the latest update on the 05/09/2019, my Galaxy S9 64 GB unlocked phone keeps coming up with the message: 'IMS Service Keeps Stopping'. The phone is also now much slower and less responsive. I have done a hardware reset of the phone and I have also wiped the cache partition and the cache and data in IMS Service and IMS Settings to no avail. Is there a bug fix/update coming soon?
I managed to use the US instant chat messenger by using a VPN changer and making my country USA. They have no answers for us, either take it back into a help centre or send it to one. No idea when a new patch is coming out, no other solution, pretty dreadful customer assistance. I am going to try Flash S9 Firmware to the previous update as I have seen in other threads that this has worked.
Samsung have essentially bricked grey market phones in Australia and New Zealand. Their sluggishness in fixing the problem is in-execusable. I think news of this problem perhaps needs to be spread a bit wider. If it became more widely known in the US that their unlocked phones won't work overseas I suspect there would likely be a significant backlash against Samsung and loss of sales, and Samsung might try a bit harder to fix the issue.
Currently the only fix is to re-install the previous firmware version for your phone and somehow disable automatic updates to stop the latest version from installing again (I'm unsure how to acheive that).