"4.3 Restrictions. Customer will not, and will not allow third parties under its control to: (f) use the Service to operate or enable any telecommunications service or in connection with any Application that allows End Users to place calls to or to receive calls from any public switched telephone network;"
Source: https://developers.google.com/appengine/terms
A question was asked about this a few months back and a GAE PM basically said use it at your own risk. Here's the thread: https://groups.google.com/d/topic/google-appengine/c8u71g5Cc...
Hopefully, this is a signal that they want to sort this out...