I follow a simple rule. If I'm looking for a written conversation, I say hello followed by whatever I want to say next in the same message. If I want to call someone, I say hello and ask them if they are available in the same message. I avoid situations in between because they are usually highly entropic and confusing. And sending a single message works, so there's no need to fix it.
Use "Hello, not urgent, please ping me when your are a available for a live chat. I have a complicated question about $topic."