Dear contractee,
your description is short, but clear. You need a Asterisk configuration/modification that does a speech recognition with incoming RTP at early audio stage (before call is answered) and if the incoming audio includes a predefined message, then the call should be processed. A bit tricky, but feasible.
About me: i work with Asterisk since 2004 (since Asterisk version 0.98) I worked three years for the german PBX provider nfon.net. The biggest cloud-PBX provider in Europe as asterisk developer. Now i work as CTO at "HTWerk". A company that has developed a android based VoIP phone.
I would be glad to work on this project. I is a interesting and challenge task.
Best regards
Kristijan Vrban