How soon after sending out an initial email to investors do I follow up with a phone call?
It doesn’t matter.
If a VC is interested, then after your meeting, he will follow up with you.
Checking in via email in a few days is a good idea, it will keep things top of mind. Do that, by all means. And you can try to create some urgency with the follow-up. And/or ask if you can get them references, diligence materials, more info, etc.
But really you don’t have to do anything.
VCs typically meet from 50–200 start-ups each per year, plus they have to meet with other start-ups that their partners bring in. So all told they may “see” 300-400+ new start-ups a year.
They know when they see one they want to do.
And they follow up.