Should I send a follow-up email or call again (to potential PhD supervisor)?
I think it would be better to wait till the end of Wednesday, and then send a follow up email. It could be that he forgot to call or may be quite busy, given the (small) delay in his previous response. For the same reason, calling directly may not be a good idea.
In addition, do check that you did not miss any call or were unreachable at some point during the day. If so, you could mail immediately, apologize, and request to reschedule.
For future reference, it would have been better if you had mentioned your time constraints for the next several days and included your phone number in your first email that you sent on Monday. This would have made the scheduling much easier and the rest of the emails and the confusion after could have been potentially avoided.