Sending is easy now. There are a number of web-based portals for sending. Receiving is a much bigger issue.
SMS is generally a cell phone thing, so the PLC would have to become a cell phone. I'm sure there is a market for that, but I'm equally sure the development cost and effort would be outside what we are currently able to commit to the effort.
There are some non-phone SMS facilities out there. That might be a starting point. It may be possible to pull content from one of them via HTTPCMD, but that would be polled. MQTT is a better option if there were some form of SMS to MQTT gateway. Don't know if that exists though.