I'm absolutely sure. The app is connecting to an Asterisk owned by me, so I can see the status of the account. I made these steps:
1. Activate the on-screen SIP debugging in my Asterisk with "sip set debug on". This prints any received SIP message on the screen.
2. Launch the Telephone app.
3. The app log shows that is registered to the server, but the UI status remains "Offline". No SIP messages printed on the Asterisk console. "sip show peers" shows that the peer is not connected.
4. Manually change the status to "Available".
5. At this moment, a lot of SIP registration messages appears in the Asterisk console. "sip show peers" shows that the peer is connected now. Calls are working now.