this is a pure asterisk issue. your phone is not being run through the vicidial dialplan, so it is not able to utilize the dial services of vicidial.
if you created an sip phone with credentials that send the calls through [incoming-internal] (which appears to be what you have done according to your post),
Cannot find extension context 'incoming-internal'
then you must create an [incoming-internal] context with a dial plan in it.
if you are completely unfamiliar with this, you could try sending the calls to [default] instead of [incoming-internal] when defining your incoming sip phone.