I have a main vet. And he is a busy man because he's GOOD. If I ever failed to adequately describe the seriousness of him coming out, he certainly isn't going to when 3 other people are calling with REAL emergencies.
As far as I'm concerned, not calling another vet is absolutely ridiculous. We all want to have our own vet, but we are not the only client on the list. Regardless of the reason why he wouldn't come out, it is YOUR responsibility to ensure your horse has adequate medical attention, not his.
I'm pretty sure you can't sue a vet for choosing not to come out, regardless of the reason. He's not on contract, he's not your employee, and if a vet was sued every time a horse died because they couldn't make it out, we'd have some mighty poor vets.
No vet can properly assess over the phone, it's nothing more then advice, exactly as you said. I could tell my vet my horse has a 6" gash on her shoulder oozing blood, and he will advise me to just bandage it, meanwhile she has arterial spray and I have not described the injury in a way that makes him understand the story. It is MY job as a responsible horse owner to know when my horse needs veterinary attention, and obtain that attention, regardless of who I have to call.