Prodding Channel Failed

Hello ladies and gentleman, as a newbie I am quite lost with all SIP, Asterisk, FreePBX and other terms, but I am trying to learn.

Question:

I installed Asterisk/FreePBX on my laptop (running Ubuntu 14.04.01 Desktop edition), started up and registered 2 (hard) SIP phone (Aastra 55i, on the same network), but when I try to give a test call from one phone to the other I keep getting “Your call cannot be completed as dialed. Please check the number and dial again.”

This is my log:

[2014-09-25 15:56:38] WARNING[16148][C-00000008]: channel.c:4874 ast_prod: Prodding channel ‘SIP/demo-bob-00000008’ failed
== Spawn extension (from-internal, 6001, 8) exited non-zero on ‘SIP/demo-bob-00000008’
– Executing [h@from-internal:1] Hangup(“SIP/demo-bob-00000008”, “”) in new stack
== Spawn extension (from-internal, h, 1) exited non-zero on ‘SIP/demo-bob-00000008’

Your help is highly appreciated, by this lost students’ side.
Thanks