Cisco additional unit Cisco SPA500S Console is available to use with Cisco VoIP phones SPA50x series:

1. Connect your console to VoIP phone. 

1: Screw holes;

2: Peg holes;

3: AUX IN port;

4: Cable slot;

5: AUX OUT port.

2. Launch a web browser on a computer that can reach the phone on the network.

3. Enter the VoIP phone's IP address in your web browser address bar.

For Cisco SPA504G there will be opened web interface:

4. Click on the link Admin login, then advanced. Click to Attendant Console tab to get access to console settings.

5. Set fields default, as shown on screenshot (General).

Attendant Console Call Park Code: *68

Server Type: Asterisk
Attendant Console Call Pickup Code: *8# 

6. Settings for Unit 1 (the first console).

We can provide console functions: reception of call, "subscriber is busy" signalization, subscriber extension of short dialing.  

Unit 1 Key 1 field must be filled by text: "fnc = sd+cp+blf;; vid=1".

If SIP line for outbound calls is registered on tab Ext1, then VID parameter should be set as 1.

If it is on tab Ext2, then VID should be set as 2.

You can setup other extension similarly. See example for Ext1 on screenshot below.