You cannot add "Cisco Unified Wireless IP Phone 8821 - Cordless extension handset - with Bluetooth interface - IEEE 802.11a/b/g/n/ac (Wi-Fi) - SIP - 6 lines" to the basket because the product is out of stock.
Cisco IP Phone 8841 – VoIP phone – SIP, RTCP, RTP, SRTP, SDP – 5 lines