Change "asterisk" from SIP header


I’m trying to configure my asterisk as stand-alone voice mail server. It’s working good if I register my phones in asterisk, however if I use just as a voice mail for other PBX when I leave a message, my asterisk is sending an NOTIFY to other PBX, but unfortunately my voice mail key is not flashing… I think is because I’m receiving my NOTIFY with letter I would like to change “asterisk” for “1000”. Is there a way to change this asterisk from the image bellow: