Backgroup multiple digits input?

Hi all,

Basically I have the following in extensions.conf:

exten => s,1,Answer
exten => s,2,Wait(1)
exten => s,3,DigitTimeout,5
exten => s,4,ResponseTimeout,10
exten => s,5,Background(myintro)
exten => 4,1,Playback(success)
exten => 4,2,Hangup
exten => 510,1,Playback(success)
exten => 510,2,Hangup

If I press 4 during the intro, the success message is played. If I press
510 during the intro, I get an invalid extension. Am I missing something
obvious here? I want to handle multiple-digit extensions during a Background…

Thank you, flo_turc !