They can obviously communicate with Starfleet. "Subspace frequencies" or whatever they called it. Presumably personal and not just official communication would happen the same way. It's just not something that was top of mind when those shows were made. Long distance phone calls were still something you paid for at a substantial cost per minute. The idea that you'd be casually chatting with friends light-years away just didn't occur to anyone.
Not always. Lots of episodes they are well out of range of communications with Starfleet. They have even mentioned not getting a response from Starfleet for weeks in a number of episodes.
Except in Voyager (where due to the nature of the premise they had to be _somewhat_ consistent) this was entirely plot-driven. It could be anything from "real-time comms halfway across the galaxy" to "we'll get a reply next week" to "no contact at all". Occasionally this even varied within the same episode.
