How to register a trunk in asterisk realtime + mysql?

Question

MySQL

Hello, I ask you not to judge strictly if the question seems stupid to someone and thank you in advance for your answer.
How to register a trunk in asterisk realtime + mysql ?
The following situation, the trunk is registered if I set the callbackextension parameter to a value, I set the internal number, for example, 101. If I remove the value and leave the field empty, the trunk registration does not work. I can not find a complete manual for Asterisk Realtime. Generally it would be desirable that the entering call left on a context.
Thank you in advance for your response. I ask you not to send to Google for 8 hours already in search.