[asterisk-users] Assigning an extension to a roaming phone

[Axelle]

Hi,
I'm trying to automatically have the dialplan assign an extension to a
roaming phone on my network.
I tried the following without success:

exten => 3001,1(readop),BackGround(beep)
exten => 3001,n,Read(digito,vm-youhave,3)
exten => 3001,n,SayDigits(${digito})
exten => 3001,n,Set(ROAM=${digito})
exten => 3001,n,Set(DB(roam/ext)=${digito})
exten => 3001,n,playback(vm-goodbye)
exten => 3001,n,hangup
exten => _4XXX,1,Set(ROAM=${DB(roam/ext)})
exten => _4XXX,n,dial(SIP/${ROAM})

The idea was that the roaming phone first dials 3001, sets a 3 digits
extension (eg 123) and then I supposed that 4123 would work. But it
does not.
I am unsure about the 2 Set lines.
Can anyone help?
Regards