question

It's just not responsive enough at all. Text messages take forever to come in half the time, and every now and then, a new message is visible in the quick one-line preview in the Messages overview but when I enter the conversation, they don't show up for a very long time. Calls are ridiculously unreliable. They often hang up within seconds of placing them, and I'm in an area with decent service. Receiving calls most often happens through the phone forwarding because that's unreliable through the app too. I am using an iPhone 4S running iOS 9.0. It had many of the same issues in 8.4.x before.

Some people complain that calling my main number sends them directly to voicemail instead of ringing me and we've just given up

This sounds like a network issue.  First be sure that you have the most recent version of the mobile app, which is 7.4. at this time. 

Next,  you'll want to do a network speed and quality test.  There are several free sites that allow you to do this on mobile devices. Perhaps try www.freeola.com   You'll fine both speed and quality tests on the site. 

You'll want to run the test at least 2-3 times on your wifi connection and then again 2-3 times on the data plan. If the speed is too low, or there's any packet loss or excessive jitter, then that will cause problems. 

Mike