I did switch from a DAHDI line to a SIP trunk and recorded a voice notification informing about the number change. After the voice notification the call needs to be killed but Hangup ddoes not work for DAHDI on my PBX.
Easy fix is to use Congestion. See dialplan below.
exten => s,1,Answer
exten => s,n,Background(greeting_number_change)
;exten => s,n,Set(TIMEOUT(response)=1)
;exten => s,n,Set(TIMEOUT(digit)=1)
exten => s,n,Congestion