There is good reason to want to immediately contact a customer when you see that they have opened an email, clicked a link, or replied back to you. Replying within 5 minutes increases your chance of closing the sale by 9x (according to InsideSales.com). So yes, your instinct is a good one.
But if you call and say something like, "Hey, I saw that you just opened my email, so I thought I'd call," then you are definitely going to creep them out.
A better way to start is to say, "Hi Ms. Smith, I was calling to follow up, how have you been?" Ms. Smith will likely tell you that she had just looked at your email, so what a coincidence that you would call! But discussing the wonders of modern technology with her would be distracting right now. Remember, you care about this person, and you want to see if you can help them make a good decision. Take care of your customer. There's never been a better time than right at this moment to do that!