I found that it's not just cell phone providers. It's a problem with VOIP too. Someone calling me with Ooma had the same problem.
I have no reason to think it's not a problem with landline too. All I know is that it doesn't happen 100% of the time. I've called the number with my own cell phone and had it ring, so placing one call from a landline doesn't prove anything except that it doesn't happen 100% of the time. But I already knew that.
Sonic's "I tried it once and it worked so it's somebody else's problem" is not the way to handle a problem that doesn't always happen. When they determine that it's not a problem by claiming that their system worked properly one time, especially when they didn't even call me to test it, makes no sense.
So the vendor is Sonic. It's nonsense to say that every vendor except for Sonic has a problem but only when calling my specific number. If those other companies were all having problems, then other Sonic customers would be hearing from people who called them, since Sonic thinks that all other providers have defective equipment.