Photo by Apple
You will be excused if you sometimes forget that iPhone can be used for regular old phone calls. But our guess is that Apple still considers phone calls to be crucial for the iPhone, which explains why the company pushed out iOS 12.3.1
The other two errors resolve issues with Messages. The first error can allow messages from unknown senders to break through even when you have enabled the Unknown Senders setting in Settings> Messages> Unknown and Spam. And the other one can make it harder to report message mail by preventing the Report Trash link from appearing in message threads from people you've never corresponded with before.
Unusual for an iOS update, there are no security fixes. 
The iOS 12.3.1 update weighs in at 77.9 MB on an iPhone X and 394.5 MB on a 10.5-inch iPad Pro. We have not heard of any issues with it so far, so we would urge anyone running iOS 12.3 on an iPhone to install it right away to take advantage of the VoLTE repairs. For users of an iPad or an iPod touch, there is probably no harm in waiting a little longer, unless you regularly receive text from unknown senders in Messaging.