I was alerted to the fact that subscribers using AT&T who have SMS contacts set up are not receiving SMS text messages. I dug deeper into this issue and found that the messages to <number>@txt.att.net are bouncing when sent from my server. Some research on the Internet resulting in several posts on AT&T forums with others complaining of the same issue. Some posts date back to 2018! Apparently AT&T severely rate limits messages sent through this service and delays delivery or in my case refuses to relay the messages at all.
Of course, none of this works very well for sending time-sensitive weather alerts. Unfortunately I am unable to revert back to the previous behavior of sending SMS text messages. My suggestion is for those users who wish to continue receiving WxNotify alerts use a different & more reliable contact method, such as the new options Discord, Pushover, Slack, or Telegram. Please refer this article for setting up contacts.