Asterisk 1.4. voicemail

i have such dialplan

exten => 2490909,1,Playback()
exten => 2490909,2,ChanIsAvail(24901)
exten => 2490909,3,Dial(SIP/24901)
exten => 2490909,4,Voicemail(2490909@taxi,b)
exten => 2490909,5,Hangup()

it`s not working

i need when the line is busy send voicemail to email with sound file

Chanisavail requires a full technology/resource address. You also have to test its result. You will need to set a call limit on the resource.

Even without the Chanisavail, your dialplan will go to voicemail if the phone returns a busy status, or a call limit condition indicates that it is busy. It will not do so if the call is simply ringing on the second line.

Given that call limits have problems when calls can be originated in both directions, you may need to do an explicit test for in use, rather than relying on the implied one for busy.