How can I get my Mac to automatically connect to a given network whenever it becomes available?
It sounds like you may have the phone-hotspot network prioritized over your home wireless network. You can change this in System Preferences -> Network pane -> Wi-Fi service -> Advanced button -> Wi-Fi tab -> Preferred Networks list. Just click and drag to put your home network above the phone hotspot, and when it sees both it'll automatically choose the home network.

Note that this will not trigger a change whenever the home network comes into range, so it's not quite what you asked for. But it will re-scan and re-choose whenever it wakes from sleep, so if you put it to sleep on your way home (i.e. close the lid), it'll re-scan and choose the home network when you wake it back up.