[HELP] Dial plan for checking voicemails from outside

I originally had the following in my extensions.conf:
exten = 650,1,VoicemailMain(${CALLERID(num)},s)

so users at their desk could push the message button on their phone and go straight to their voice mailbox.

Now, when users call from outside and ask the receptionist to transfer them to extension 650, it dumps them in the receptionist’s mailbox.

How do I let them bypass mailbox/password prompts when they call from their own phone, but prompt them if they’re calling from outside?