asterisk users - Jan 2004 - Not sure what I'm looking for to ask correctly.

I have an * box at my home office.  I have my GS 101 at work that registers
fine with it.  However, when I try to dial voicemail from the phone @ work,
it "sometimes" doubles the digits I enter.  Example:  I designated
8500 as
my voicemail, so when I dial it...and it prompts for mailbox/password and I
enter 5000 password 1234.  It will double the digits.  I see it when
watching the CLI, mailbox:550000 password:11223344.  The speed is around
115ms from the phone to the * box.  What do I need to do to prevent this?
Sorry if I don't know the exactly terminology.

Regards,
Joe