This article describes how to block incoming calls based on the phone.

Polycom phones can Auto-Reject a call to Voicemail & Auto-Divert calls to an outside number by creating a Phone Book contact.
TIP: If you Auto-Divert calls backs to the Caller ID of the abusive caller, the call will disconnect.
This enables you to prevent an abusive caller from leaving voicemails.

Vertical desk phones can only Auto-Reject calls to Voicemail.

--Polycom 2-line
--Polycom 4-line
--Polycom 6-line
--Polycom VVX500
--Polycom Conference

--DPS-F Vertical deskphones (Auto-Reject calls only) 
--DPS-V Vertical deskphones (Auto-Reject calls only) 
--RTX Cordless Phones: No call blocking

--Cisco SPA303G, SPA504G, and SPA525G2 (Can only block Anonymous calls)
--Cisco Cordless Phones (Can only block Anonymous calls)

--Yealink W52P DECT phones (Use blacklist feature)
--Yealink Desk Phones (Use blacklist feature)