You might consider having a SIP provider do the operation instead of your OBi. That is not to say that the OBi solution will not fill your needs.
Anveo has a system for incoming calls that lets you build a flow chart of what happens when a call comes in. I use it for home use, but its really aimed at the commercial type use. It is called "flow control". Look it up as part of your research. It can speak what you want, and can route calls to phones, messages, or voicemails based on the responses of the callers. It can also behave differently based on the phone number being in the address book, or even looking at the first part of the number, such as area code.
I am confident that other providers have the ability to do similar things, but I am familiar with the Anveo offering. It is easy to modify without having to get help.