This is obviously a bot (let loose by a human of course)
Simple bot challenges like turing tests and captcha work fine for simple bots, but there are ways around it for more advanced bots that use captcha completions services.
Captcha completion services can be as simple as outsourcing the task of completing captchas to an offshore company for a few dollars per thousand captchas completed.
A better idea for the devs might be post rate (thread creation) limiting and sandboxing the suspected spammer's username until it is reviewed manually by an admin.
Sandboxing here refers to allowing the spammer to continue to create threads, and see those created threads as if they were live, yet no other user would see those posts until the sandbox review is completed successfully. This fools the spammer into thinking that the bot is still functioning as it should. Of course it is easy to see from another account whether these posts are actually live.
Just my 2c
Edit to add: by the way, the goal of this particular spammer is not to get people on this forum calling the number, it is simply that google indexes this forum often, and these phone numbers will show up as relevant to the associated keywords and ranked higher in searches as they appear on numerous high quality, high authority domains all over the internet.
Taking these down quickly or preventing them from being shown in the first place is exactly what the spammer is hoping won't happen.