An email cannot trigger\execute an IFS process directly.

In order to bring such integration into action, the phone system should either call an IFS service (WebService or BizAPI) or use the standard IFS integration broker.

You can try this using an IFS Webservice (RESTful API)

• DevStudio: Generate an IFS projection (REST endpoint) with an action which  accepts TEXT and BINARY types.
• Generate code, add case creation logic to the corresponding procedure (not sure but you can try simultaneously attaching the binary to the docman as well)
• Call the generated endpoint via the phone system, by providing call information plus the audio recording in payload

Using IFS standard integration (PLAP)

• From the Phone Service, generate and output a text file (which contains the case info) as well as the audio recording to a network share which is accessible to IFS Application User\Server
• Generate necessary IFS Connect objects to address every new file created in that network location
  • An inbound Routing Rule (to read the file and call the routing address) and Routing Address (to execute your PLSQL logic which creates the case and attach the file)

This hopefully outlines an in-house approach to achieve the needful. 

Hope this helps.