How To Install Voice Over IP

A step-by-step guide to moving your customers from PBX to VoIP

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The lines between voice and data have been blurring since the first acoustic modems literally screamed a stream of bits across the telephone wires. Now the tables have turned. Voice traffic can be easily packetized as data.

Fully global, end-to-end voice over IP (VoIP) is not yet practical for all organizations. But intra-office VoIP hardware can be deployed in data routers and managed with the same tools used to maintain the data routers. With VoIP, data and voice are married on a single wire. The result: network complexity is reduced, and accountability is consolidated.

Also, with VoIP it is the phone itself, not the physical wire (as with traditional PBX systems) that establishes the identity and phone number of the caller. It does this via its MAC address, a unique, distingushing serial number burned into a network adapter. This can be an important distinction and a key feature for fluid office environments, especially where PCs are used as "soft phones." The extension number remains associated with the hardware, not the location, so as people move from cubicle to office to conference room, they remain reachable at the same number, transparent to intra-office callers as well as the outside world.

Especially for green-field sites and existing offices that are undergoing a massive IT overhaul, VoIP offers single-wire simplicity and a one-stop management solution for all voice and data traffic.

Leading telecom and networking manufacturers, including Nortel, Siemens, and Cisco, are eagerly introducing business-class VoIP across their product offerings. So you don't have to look far to find an OEM willing and able to help you supply your clients with a unified voice/data environment. We will illustrate our recipe with Cisco gear, though you can of course use equivalent products from other vendors.

This recipe describes a green-field implementation of a small-ofice VoIP network. At its completion you will have an operational combination voice and data router, as well as several VoIP phones configured for office workers. There's no need for dedicated racks and cable nests just for voice service.


  • Cisco 3745 router. Other 3700-series routers, such as the 3725, can do the job, but the 3745 offers more room for expansion and additional DSPs (digital signal processors) to handle heavy voice traffic.
  • Cisco DSP network module. Models such as the NM-HD-2V provide the necessary hardware to handle real-time conversion of multiple voice/data streams.
  • Cisco IOS IP Plus Software 12.2.15ZJ. Cisco's IOS software provides the IP routing and data traffic backbone necessary to operate the router.
  • Cisco CallManager Express. Known as CCME to its friends, this is the core voice-handling software. It manages user groups, dialing permissions, and call routing. While the "medium" license pack supports up to 48 users, CCME can be licensed for as many as 120 users. Larger sites will need standard CallManager, which scales up to 30,000 users.
  • Cisco Unity Express network module. The CUE's most important job is to provide voicemail for the office.
  • Cisco 7912G phones. You will need one phone for each employee on the network. The 7912G is a solid, standard-issue VoIP phone. A two-port switch is wired in, so only one cable needs to be run to any work surface. The user's PC can take its data feed directly off the phone.
  • Cisco phone, model 7940G or higher. This is for the boss or bosses. You will want one phone for each executive. These phones provide larger screens, more buttons, and access to more than the two phone lines.

  • T1/FXO network module. Depending on the type of connection your client has purchased from its telco, you will need a connection-specific network module from your networking-equipment vendor. Cisco's model VIC2-4FXO is a representative module from this category.
  • Line Powered Ethernet Switching, model NM-16ESW-PWR. Highly optional but extremely slick, this 10/100 switch provides wireline power to the VoIP phones. It eliminates the need for wall adapters, not coincidentally making the whole implementation look more like a traditional PBX arrangement.
  • CAT 5 cabling, as needed. Whether you go with the basic blue cable or something less eye-catching, you'll need a lot of it. The good news is that each desk only has to be wired once. That's a major improvement over the wired office of the '90s with both telephone and Ethernet wire running from floors and ceilings.

From Start To Dial Tone

Now that we have our components assembled, let's start building the VoIP network. Here are 11 steps to VoIP success:

1. Before any internal work is done, you or the customer must arrange an outside connection with the customer's telelcom-services provider. Fortunately, nothing is different for the customer or the provider at this step. As far as the telco is concerned, a VoIP network looks exactly like a traditional PBX. All you have to do is find out the type of connection (T1, FXO, etc.) and order the proper network module to suit.

2. Assemble the VoIP gear. All the parts listed above, including the CallManager Express software, may be purchased preinstalled from Cisco. Our contacts at Cisco dubbed this configuration the CISCO3745-V-CCME-A bundle. All components will come installed with generic setup steps already performed. Additional modules, such as inline Ethernet power, may require field installation.

3. Connect the FXO/T1 network module to the central office trunk provided.

4. Using your Cat 5 cabling, deploy the IP phones. Remember that each phone has a two-port switch, so most workspaces will only need a single line run from router or workgroup switch to the desktop. Link each phone with the router, then connect the desktops to the phone's other 10/100BaseT port.

5. Turn on the central timekeeper. The VoIP network relies on a central timekeeper. Cisco ships routers with Network Time Protocol (NTP) service disabled, but it should be turned on for smooth operation and easier configuration of new phones.

Here's the command sequence to set the clock and get NTP running:

clock timezone zone hours-offset

clock summer-time zone recurring

ntp server ip-address

6. CCME can automatically discover and configure phones on the network. Launch telephony-service setup from an IOS CLI (command-line interface) prompt. This invokes a text-based Setup Tool Wizard. Before running the Wizard, be prepared to answer the following questions:

  • Should all phones be configured with DHCP? (If you require multiple DHCP pools, the default DHCP configuration can be edited later.)
  • How many phones will the system be able to support? Let purchased licenses be your guide. CCME can automatically detect and configure new phones as they are added to the system at a later date.
  • Are phones to be single or dual-extension?
  • What is the starting extension number on the network?
  • Is direct inward dial available for all phones? In other words, does each phone have a unique outside number? If not, or if only some do, then CLI configuration is required.
  • Should unanswered calls be directed to an answering-service number?

Here's the CLI summary for this step:

telephony-service setup



show running-config

7. Configure a basic, fully consultative transfer system. Most offices will want some form of call transfer. Both blind (immediate connection) and consultative transfers are supported. With consultative transfers, two internal users can confer about the transfer before the caller is reconnected.

To configure a basic, fully consultative transfer system, enter the following sequence in the CLI:


transfer-system full-consult


The transfer-pattern command may be used before the exit to include a range of outside dial numbers as valid transfers. Without it, only extensions within the VoIP network are valid transfer destinations.

8. Save clients expense and embarrassment by excluding all 1-900 number calls using a simple CLI sequence:


after-hours block pattern 16 .1900 7-24

9. Set the CLI aside except for elaborate configuration problems. CallManager Express (CCME) relies heavily on Web-based GUI configuration. If the router was purchased new in step 1, its HTTP server is disabled by default. Coordinate with the system administrator to enable HTTP access for admin accounts. The CLI command sequence will be of the form:

ip http server

ip http path flash:

ip http authentication

Next, activate the GUI itself. The command sequence is of the form:


web admin system name username




The CCME GUI may then be opened in a browser window, pointing to Substitute the IP address of the router as appropriate.

10. Use the CCME GUI to create any additional, necessary administrator accounts. Also use it to create user accounts for each phone, so employees are authorized to configure voicemail and perform other basic tasks.

11. In the CCME GUI, verify that users have their desired extensions. The phones were auto-discovered and numbered sequentially with the setup wizard, but some clients may require vanity extensions for certain users. The Configure/Extensions screen allows extensions to be created, deleted, and reassigned to a different MAC address. The Change Phone screen also allows the administrator to set up certain personal features for the user, such as pre-populating speed dials.

At this point, you should have a happy, healthy IP telephony environment, with dial-tone to the outside world and all the convenience of a one-line, one-box administration. The CCME GUI is loaded with complex features, and even more esoteric configurations are possible through the use of the telephony-service series of commands available from the CLI. It will make that wheezing AT&T Merlin setup being replaced look positively Stone Age.

JASON COMPTON is a technology writer who has covered topics ranging from 8-bit entertainment to supercomputing for more than a decade.

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