The only way I've been able to get this to work so far is by having the following entry in the replacement blank in Smart Dial/Dial Plan:
Since I have _99 as the Pattern, when I dial 99#, the call gets routed to callcentric and **257*010500 gets prepended, while 99 is stripped off. This works, i.e., it gets me Comedian voicemail and the prompt to enter a mailbox number, but that doesn't seem to be exactly what you were describing.
Any further tips on implementing this in a more elegant way?
Thanks,
James
PS
works too.