I'm a private music teacher. My number is is given out by music stores and other places. 2018 was fine, 2019 is becoming a problem. Including yesterday, I lost 3 communications in the last 2-3 weeks that I know of.I realize that your situation is different from mine, because you use your line for business, but I am glad that I haven't yet had to do business with someone who lacks the basic intelligence to ask whether my number is a cell phone or not. Or at least, I have not suffered due to non-receipt of such messages. Nobody's asked me "Didn't you get my text?". Of course, now that I've said it, it will probably happen!
I spent about 4 hours yesterday doing research and making phone calls to companies that might have a fix for my situation. Once I sort through the info and mess, I will post back and let you know what I've learned. Unfortunately, some of these solutions are expensive enough that they defeat the purpose of dropping my AT&T landline (which went up to $95 a month here in California with taxes when I cancelled it).
Did I mention earlier that my T-Mobile cell phone doesn't get reception where I live? I'm working on that today.