Enabled Twilio in the example app settings
Fixed #54 -- Mitigate voicemail hack

See also [this blogpost][1] which uses the victim's voicemail
service to access OTP tokens. When the call goes to voicemail, the
tokens can be access through a vulnerable voicemail service (e.g.
requiring no PIN). Then the attacker will have access to the tokens
and is able to login. When requiring interaction (e.g. pressing a
button), this type of attack can be disarmed. Major services like
Google and Yahoo are still vulnerable to this type of attack.

[1]: http://shubh.am/how-i-bypassed-2-factor-authentication-on-google-yahoo-linkedin-and-many-others/