Voicemail : Redundant value boxnumber is removed

Voicemail : boxnumber under extension{i} was redundant and is removed.

Earlier "config_foreach configure_voicemail extension" was used , which added 4(one for each call) instances of "VoiceMailMain" in extensions.conf.Since we can get value of voice_mail_enable from line ,using line to check value of voice_mail_enable.

Voice mail deposit and retrieve is tested. With multiple line changing voice_mail_enable (1/0 )functionality is tested.