When one method of communication fails, try another one
You've tried sending an email but it didn't have the desired result. If you're still intent on achieving your goal, you should try a different method of communication, in this case most likely a phone call.
[Greeting], this is [user82123] calling. Last week you sent me an email to say that you'd like to have a more in-depth conversation, when would be convenient for you to do that?
With a little luck, the answer might even be something like "Hey, thanks for calling, in fact I have some free time right now if that works for you" and you can get the whole thing wrapped up right there and then!
This concept is one to remember for any point in your professional career: don't keep using the same method of communication over and over again, if one doesn't seem to be achieving the desired result, try a different one. This usually means starting with chat/email, then working up to a phone call, and then just going to the other person physically.