How to Configure a Cisco ISR4331-V/K9 Router for Small Business VoIP & Internet Access

How to Configure a Cisco ISR4331-V/K9 Router for Small Business VoIP & Internet Access

Looking to quickly get your Cisco ISR4331-V/K9 router up and running with a dynamic WAN IP, internal LAN, NAT, and basic Call Manager Express (CME) telephony? This step-by-step guide covers WAN setup, secure NAT, internal DHCP on 192.168.1.0/24, and a basic working CME configuration with two sample extensions—perfect for clinics, startups, and growing offices.

Prerequisites

  • Cisco ISR4331-V/K9 with appropriate licensing.
  • Basic terminal access (console, SSH, or Telnet).
  • Two IP phones (or softphones) for extensions.
  • Placeholder MAC addresses (replace with your phones’ actual MAC).

Step 1: Basic Router Setup and DHCP WAN

hostname reeftel-router
no ip domain-lookup
!
interface GigabitEthernet0/0/0   ! WAN interface to ISP
  ip address dhcp
  ip nat outside
  no shutdown
!
interface GigabitEthernet0/0/1   ! LAN interface
  ip address 192.168.1.1 255.255.255.0
  ip nat inside
  no shutdown
  • The WAN interface gets its IP via DHCP from the ISP.
  • LAN interface is statically set as the default gateway for clients.

Step 2: Enable DHCP Server for the LAN


ip dhcp excluded-address 192.168.1.1 192.168.1.10
ip dhcp pool LANPOOL
  network 192.168.1.0 255.255.255.0
  default-router 192.168.1.1
  dns-server 8.8.8.8 
  • Ensures the router and servers don’t get assigned from the DHCP pool.
  • Devices on LAN get IP addresses automatically in the 192.168.1.0/24 range.

Step 3: Configure NAT to Allow Internet Access

access-list 1 permit 192.168.1.0 0.0.0.255

ip nat inside source list 1 interface GigabitEthernet0/0/0 overload

  • Allows PAT (port address translation) for multiple LAN devices over one ISP IP (dynamic or static).

Step 4: Basic Call Manager Express (CME) Setup with Two Extensions


telephony-service
  max-ephones 2
  max-dn 2
  ip source-address 192.168.1.1 port 2000
  auto assign 1 to 2
  create cnf-files
!
!
ephone-dn 1
  number 101
  label NurseStation
  name NurseStation
!
ephone-dn 2
  number 102
  label Reception
  name Reception
!
ephone 1
  mac-address 000A.BCDE.0001
  button 1:1
!
ephone 2
  mac-address 000A.BCDE.0002
  button 1:2
 
  • This config sets up two extensions: 101 for "NurseStation" and 102 for "Reception".
  • Replace the MAC addresses with those from your IP phones.
  • Phones will register and receive their extension and label after reboot.

Step 5: Save and Reload


write memory
reload
  • Always save your configuration!

Final Thoughts

This configuration will get your Cisco ISR4331-V/K9 up and running with:

  • Reliable, automatic WAN and LAN IP management.
  • Secure NAT for Internet access for your entire office.
  • Two functional VoIP extensions with Call Manager Express.

For more advanced features or troubleshooting tips, explore Reef Telecom’s knowledge base or get in touch with our experts to make your network and voice setup as smooth as possible.

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