A Security Analyst Wants to Harden the Company's VoIP PBX

A security analyst wants to harden the company's VoIP PBX. The analyst is worried that credentials may be intercepted and compromised when IP phones authenticate with the BPX. Which of the following would best prevent this from occurring?

A) Implement SRTP between the phones and the PBX.
B) Place the phones and PBX in their own VLAN.
C) Restrict the phone connections to the PBX.
D) Require SIPS on connections to the PBX.