This configuration cost me 2 day of work. Now I can make an receive calls thru Extension IP Axtel http://www.axtel.com.mx

You need to create a trunk as following

[axtel]
fromdomain=axtel.tel
fromuser=area_code+number+"ip" (Ej. 8115151515ip)
host=extip.axtel.net
insecure=very
secret=your_password
type=peer
username=area_code+number+"ip" (Ej. 8115151515ip)

register => area_code+number+"ip"@axtel.tel:your_password:area_code+number+"ip"@axtel/area_code+number