I changed it to this, and it works:
${VM_NAME},\n\nThere is a new voicemail in mailbox ${VM_MAILBOX}:\n\n\tFrom:\t${VM_CALLERID}\n\tLength:\t${VM_DUR} seconds\n\tDate:\t${VM_DATE}\n\nDial *98 to access your voicemail by phone.\nVisit http://myPBXserver/ucp/?display=dashboard&mod=voicemail&sub=${VM_MAILBOX} to check your voicemail in your web browser.\n
However, is there a way to display the url as a hyperlink with regular link text instead of just a url?