Something

        exten => 8501,1,Answer
        exten => 8501,n,Verbose(0,Channel "${CHANNEL}" wants to know what extension it is)
        exten => 8501,n,Set(chana=${CUT(${CHANNEL},/,2)})
        exten => 8501,n,Verbose(0,  chana="${chana}")
        exten => 8501,n,Set(chanb=${CUT(${chana},-,1)})
        exten => 8501,n,Verbose(0,  chanb="${chanb}")
        exten => 8501,n,Set(ext=$[${chanb} + 7475)
        exten => 8501,n,Verbose(0,  ext="${ext}")
        exten => 8501,n,Playback(vm-num-i-have)
        exten => 8501,n,SayDigits(${ext})