VoIP Resources VoIP Reviews Desk Phones

Cortelco C56

by Kevin Bartley

The Cortelco C56 is an economical phone with sturdy voice calling and minimal features. Check out our full review.

Published: October 15, 2015


OnSIP Testing:star_orange_24star_orange_24star_orange_24star_orange_24star_orange_24
Voice Quality:star_orange_24star_orange_24star_orange_24star_gray_24star_gray_24
Ease of Use:star_orange_24star_orange_24star_orange_24star_orange_24star_gray_24

Quick Specs

  • HD Voice capable
  • PoE support
  • MWI support
  • 4 programmable DSS keys
  • Supports up to 2 SIP accounts


  • Solid voice capabilities
  • Easy setup
  • Low cost
  • Durable and portable


  • Middling voice quality 
  • Primitive menu system

Note: The C56 has since been discontinued. 

The Cortelco C56 is one of the simplest SIP phones we've ever reviewed. With its easy configuration and its bare bones interface, the C56 is clearly not trying to win the arms race of smartphone-esque UIs and advanced features that some other manufacturers are aiming for.

The C56 boasts two SIP accounts, HD voice capabilities, PoE support, MWI support, and four programmable DSS keys. In other words, this phone is able to make phone calls, and that's about it.

The Cortelco C56 banks on the "less is more" ethos, and it would be unfair to compare it to touchscreen SIP phones with deep feature sets. So we'll treat C56 how it wants to be treated; as a “fleet” phone meant to be deployed en masse or in contexts where only voice calling is needed.

First Impressions

Cortelco C56 Front
The Cortelco C56 boasts an economical design with no frills.

You first notice the C56's understated frame when you take it out of the box. Outside of the number pad, there are very few ancillary buttons. Hold, transfer, conference, and redial functions are the only features encapsulated as buttons. The user interface displays characters more like a digital clock from the 1990s than the interfaces of today's standard SIP phones. The C56's plastic is glossy and smooth but seems hollow to the touch.

If you bought the C56 hoping for a universe of buttoned features and an Android UI, this is the part where you frown.

But if you did your homework and you know that the C56 is meant for limited usage, you actually come to appreciate the lo-fi aura of the phone. Sure, the materials are lightweight and unvarnished, but that actually speaks to the unique portability of the C56. In fact, the C56 may be the most portable phone we've ever reviewed. If you work at sporting events or nonprofit charity drives that have to build and rebuild phone centers on a consistent basis, this could be the phone for you.  


In case you haven't noticed, the C56 is light on features. With just two SIP accounts, the phone can handle two lines at a time. This could be a problem for users who are linked to multiple SIP accounts. Cortelco is clearly not trying to win over companies with an abundant number of interlocking SIP aliases.

The C56 has three ports on its backside: one for power, one for LAN connectivity, and one for a personal computer connection. The phone is otherwise devoid of any connective ports. Cortelco shot for a simple design, and in that department, they've succeeded.

Cortelco C56 Functions
The menu system focuses exclusively on IP issues and uses a blue backlight.

Sysinfo simply displays the phone's IP address, which you will need to configure the phone, and History presents the numbers of the last few calls the phone made. The internal menu system is limited to Network, LAN, IP, and Static Net—all basic networking functions that help connect the phone to the Internet. You won't find any calendars or email clients on the C56.    

Hold, transfer, conference, and redial rarely get big attention in our reviews, but these are the only buttoned features on the phone. The hold button is actually a multifunction feature that does more than simply put the current caller on hold, depending on the context. It will assume you are attempting to dial somebody else when you put a caller on hold, instead of just holding them, or it will automatically place the current caller on hold and answer a second incoming call. Cortelco might be combining these functions together due to the small real estate on the phone, or it might just be an oddity.

The C56's lack of advanced features may make it irrelevant to sophisticated call center operations. But if you manage something like a pizza shop that only needs to take down orders over the phone, the C56 might be a stellar choice for you.

Voice Quality

Cortelco claims that the C56 is HD voice capable, and technically it is. Industry jargon aside, the quality of the C56's audio on a voice call is clear and intelligible but far from superior. It's not nearly as crisp as some of the Polycom or Grandstream behemoths we've reviewed in the past. But again, the intentions of the manufacturer have to be taken into account—they're not aiming to be the Mercedes of SIP phones.

The fact that the C56 can tout HD voice is mostly a testament to the term's deterioration as an accurate assessment of quality. Compared to landline phones, HD voice is a big deal. But weighed against the current VoIP phone market, the C56 is lags behind. Perhaps the term will be replaced by a more accurate buzzword after a certain period of time.

Compatibility Testing

At OnSIP, we put each of the phones we use through a multi-step interoperability test in which we apply ~30 test cases. Here’s an example of a test case:

Test phone calls phone B

B picks up

B puts test phone on hold

B calls phone C

C picks up

B transfers test phone to C

The call must be transferred correctly to C. B must be released correctly after the transfer. When C picks up, audio must work both ways between test phone and C. When the test phone is on hold, there is no audio between it and phone B.

We did not experience any issues during our tests with the Cortelco C56.

Phone Configuration

Step 1: Gather information for each user.

Each user has a set of credentials that will be needed to configure each phone. For each phone that you are configuring, obtain the following credentials. You can find this information in the user detail pages under the Users tab in the Phone Configuration section.

OnSIP Credentials

Step 2: Log into your Coretelco phone through a web browser.

When your phone is powered on and connected to your LAN, use a browser to navigate to the IP address of your phone (found via the "Sysinfo" button).

Select "Account 1."

Step 3. Enter your user information from Step 1.

For most setups, you will need to simply configure "Account 1." If you are trying to configure multiple users or multiple line appearances, you can repeat the steps below for each of the identities you are configuring.

  • Server Address: Domain
  • Authentication User: Auth Username
  • Authentication Password: SIP Password
  • SIP User: Username
  • Display Name: Whatever You Like
  • Proxy Server Address: sip.onsip.com
  • Enable Registration: Checked
Cortelco C56 Setup

Log back into the phone and navigate to the VoIP tab on the left. Enter the information as outlined above and then hit the Apply button. The phone will reboot.

When the phone is up, log back into the phone and navigate again to the VOIP tab. This time select Codec Settings.

Select the codecs to match our screenshot below.


Scroll to the bottom of the screen and click "Apply."

Step 4. Confirm that your phone is registered.

In the Admin Portal, click on the "Users" tab. You will see a green "online" notation next to each user with a registered phone.

Final Thoughts

The Cortelco C56 is an inexpensive and durable phone that works great for users who only need to make voice calls from a small number of SIP accounts. It fulfills the basic functions of a phone (hold, transfer, conference) and the voice quality is decent.

But it's not the only phone of its kind. The Grandstream GXP1610 actually costs less per individual unit and comes with a more interactive interface, although it seems the C56 might be cheaper when sold in bulk shipments.

The Cortelco C56 ultimately fulfills its intended purpose as a reliable “fleet” phone capable of quick and vast deployments. But those in search of the latest features and shiny smartphone interfaces need to look at the other options.

Learn more about VoIP Reviews