If the spammer can read it in the page then they can add it into their autodialer. Just because their spambot isn't able to extract the phone number automatically doesn't mean it can't tell there is a phone number there and report the page for manual extraction.
I have put the final (local) part of the number next to the respective position and the national code elsewhere. I seemed to have been worrying about nothing as I thought there were harvesters out there. A reader would pick up the first part before the rest.
I wouldn't say there aren't harvesters out there, because there are, however the chances are you are unlikely to get spammed via the phone because in the UK the telephone networks are much more restrictive about people who start mass dialling phone numbers, in fact the network operator would more than likely involve the police in cases of fraud or autodialing sales services. I know BT would dump them on one of those call blocking systems so it would be filtered.
How can I obscure a telephone number from harvesting bots (or whatever they're called) like one can with an email address? Or is it necessary?
I don't think it's necessary. I have had my mobile phone number on a website periodically for several years, and I get very few unwanted calls on it. I doubt that those are down to the website, because I was getting them beforehand as well!