Finding the line between persistent and annoying is the skill of an exceptionally good salesperson. I find that it's situational. On an attempt like this where it's the first call and I've managed to get through with an e-mail. I set the first follow-up later the next week (I usually end up sending those e-mails on Thursdays or Fridays, just the way it works out for me), if I don't get through at the time I said I'd call I'll be persistent and make sure I get through within two weeks of sending the e-mail. After that you're probably going to cross the annoying line if you become too persistent, I never really give up, though.

Figure out what works for your personality and what you feel comfortable with and go with that. Everyone is different and some sales people I know call more frequently than I do (Or could with my style) and have found ways to not cross the annoying line.

Just keep at 'er. I like to think of sales as an art of the persistent. It can be a lot of fun and intensely frustrating all at the same time! A lot like programming... hmmm... maybe that's just me.