SIP username variable (for Voicemail)

Most of my extensions are alpha (instead of just numerics) for security and readability and my mailboxes are the SIP user name. Does a variable exist that contains the SIP username? I’ve been able to get it via the following, but it’s pretty messy.

Anyone have a more elegant solution?

I think there is a variable ${SIP_HEADER(fieldname)} so you could do ${SIP_HEADER(username)} or something like that… keep in mind this isn’t exactly good authentication