Whoa, you're giving each person their own number (as opposed to, say, an extension). That means you're paying $1/customer/month even for dormant customers. Seems like a really bad call.

Other than that, I really like it. Sorta like Awesomeness Reminders, but recorded by people who love you. Design could use some work, but the signup flow was nice (not requiring any info but phone number). Though now the only way to login involves you calling me again and telling me the pin I already know, which isn't great.

Also, I'd stop using a redirect and set it up so that Appengine serves write.