I find small talk to be pointless but i recognize it is necessary if you dont have much to actually mutually talk about.
With personal experience on this, small talk is more of a general term than not (ie 'Stuff I think is trivial', which is subjective)--find what topics are common ground for you and your friend and *poof* no 'small talk'.
Well, for me anyways.
Also, I know you're set on phone-only, but take the bus anyway if your friend wants to hang out. For some reason it's become cliche to people, but it's true that face to face interaction has a richness which technology can't possibly match (yet). If it's an anxiety thing as it was for me, that's something I encourage you to work to get rid of, because it's a very positive thing to have in your life in the end.
This holds true forever. Technology can offer only so much communication for people, yet it is in communication that we subsist on and not technology. If its purely by phone, then I'm pretty much sure that the notion of 'keeping relations by approximates' fails when you talk about a
real friendship and it falls on more of your current friend being a future acquaintance. Well, unless you
really can't visit them in person, or have no other way of mutual contact than texting, then that's another story. But given that you can keep in touch with them, then its best to meet personally (when possible).
About those approximate numbers, while I'd give my own, I'll highlight the flaw in your ideals.
how often do you guys text/call a typical friend of yours that you only want to maintain relations with, typically?
That doesn't sound right to me, if you really want to be friends with someone. That, or I see friendship in a totally different way than you.
But if you really,
really want to keep a 'set number' and I really can't understand why...make it an hour or three of conversation. Time literally flies by that I can't count it due to the fun we have and the stuffs we talk about, even if those stuffs sometimes lie in the silence between us.
Why I can't understand this is why you're seemingly not going for the essence of what's in those messages, and instead aiming for the 'total time needed for a conversation', because I can't see that being the point at all in a conversation, much less in a relationship = friendship.