I Can't Call a Specific Phone Number

If anyone is still having this issue, I have a Samsung S8 and experienced the same. I tried everything but finally found something that worked for my phone. I went to "settings" and then "connections." Then I found"mobile networks" and "VoLTE calls" was enabled. I disabled it (just switch off the toggle) and I could make the call. I then enabled it and could still make the call so perhaps there was a glitch of some kind?

Hopefully this helps as it was hugely frustrating.