Detection of Voice Mail only for External Calls

Hello Everyone,

I am using FreePBX with snom 300 phones, I want to setup Voice Mail for 100 snom 300 phone but should get active for External Calls only. I mean when a user from inside the office call the extension then it should not redirect to VM. But when someone from outside(External) calls the reception, transferred from reception to the desired extension then after 3(or whatever) rings it should direct the caller to its Voice Mail.

Thanks

MJ Khan
Network and System Analyst