It is a Cisco-specific code that newer models of the ATA-186 recognize, that in theory switches them into "FAX mode". Some Vonage users have reported that dialing *99 prior to the number of a FAX machine they are calling improves the odds that the fax will go through faster and without problems. "Officially" Vonage never supported this, but actual user experience seems to indicate that use of the *99 code has a positive effect in at least some situations.
