It is necessary to obtain the Unified Communications (UC) license for the ISR4331 in order to connect 10 Cisco IP Phones 7821 to it. In particular, you would need the license for "Cisco Unified Survivable Remote Site Telephony" (SRST) or "Unified Communications Manager Express" (CME), which makes the router's telephony capabilities available. Make sure the license you select can accommodate the quantity of users and devices you intend to link, in this example, a minimum of ten IP phones.
