TelephonesVoIP Desk Phone
Cisco CP-8851 Telephone Manual
Setup, everyday use, maintenance and troubleshooting for the VoIP desk phone, with verified specifications.
- TypeVoIP Desk Phone
- LineVoIP (IP network)
- Standby power3.3 W
- Est. yearly cost$4.91
About the Cisco CP-8851
Cisco model CP-8851 is a VoIP desk phone that meets ENERGY STAR telephony requirements. The certification also covers related models such as CP-8811, CP-8841, and CP-8851NR.
It connects to your network as a Voice over IP (VoIP) endpoint rather than to an analog phone line. It has 4 Ethernet ports (2 Gigabit), so a computer can share the phone's network connection. It can be powered over the network cable as a PoE Class 3 device. The certification record also lists a USB port. The display resolution is 800 × 480 pixels.
Measured standby power is 3.3 W — about 28.9 kWh or $4.91 a year if it stays plugged in around the clock, as phones normally do.
Specifications
| Brand | Cisco |
|---|---|
| Model number | CP-8851 |
| Phone type | VoIP Desk Phone |
| Line technology | VoIP |
| Configuration | Corded |
| Power over Ethernet class | 3 |
| External power adapter | Not used |
| Gigabit Ethernet ports | 2 |
| Fast Ethernet ports | 2 |
| USB port | Yes |
| Display resolution | 800 × 480 |
| Standby (idle) power | 3.30 W |
| Off-mode power | 0.21 W |
Safety information
- Power the phone only from an IEEE 802.3af/at-compliant PoE switch or injector.
- Do not use the phone near water — for example near a bathtub, sink or swimming pool — and never handle it with wet hands.
- Avoid using any phone connected to outdoor wiring during a thunderstorm.
- Do not use the phone to report a gas leak while you are near the leak. Leave the area first.
- A VoIP phone works only when its network, power source (PoE switch or adapter) and phone service are up. Keep an alternative way to call 911 during outages.
Setup
- Connect to the network
Connect the phone's LAN port to a network switch port with an Ethernet cable. If the switch provides PoE, the phone powers up from the cable. As a Class 3 device it can draw up to about 12.95 W, so budget at least that much per port on the switch.
- Connect a computer (optional)
Connect a desktop computer to the phone's PC port to share one network drop. Note that the computer loses network access whenever the phone is unpowered or restarting.
- Provision the phone
Most business VoIP phones are configured automatically from the phone system or by your administrator. For manual setup, enter the SIP account, server and credentials from your service provider in the phone's web interface.
Using your phone
Calls and audio
- Adjust the earpiece or speaker volume during a call with the volume keys; the setting is usually remembered for the next call.
- Call quality on VoIP depends on the network. If voices break up, ask your administrator to prioritize voice traffic (QoS) or use a wired connection.
Wireless and accessory connections
The USB port can be used for supported headsets or accessories; check the manual for compatible devices.
Power use
Phones stay powered all the time, so standby power is what matters. This model draws 3.3 W when idle. On PoE, the switch supplies this power, and switch conversion losses add slightly to the total.
Care and maintenance
| When | What to do |
|---|---|
| Weekly | Wipe the handset, keypad and display with a soft cloth lightly dampened with water or a mild disinfectant wipe; do not spray liquids directly on the phone or use solvents. |
| Every 6 months | Check cords and plugs for damage, and confirm the phone is running the firmware version your administrator or provider recommends. |
Troubleshooting
Try these steps before calling for service. For model-specific error messages, consult the manufacturer's manual or your phone system administrator.
| Problem | Possible cause | What to try |
|---|---|---|
| Phone does not power on | Switch port not providing PoE or insufficient power budget; damaged cable. | Try another PoE port or a known-good cable, and confirm the switch can supply Class 3 power; otherwise use the AC adapter if available. |
| "No service" or registration failed | Network or VLAN settings; provisioning error; account not active. | Restart the phone, confirm it obtains an IP address, and ask your administrator or provider to verify the account and provisioning. |
| Choppy audio, echo or one-way audio | Network congestion; firewall/NAT settings; Wi-Fi interference. | Use a wired connection, enable QoS for voice, and have the administrator check firewall SIP and RTP settings. |
| Display or keys unresponsive | Temporary software fault. | Restart the phone by unplugging and reconnecting the network cable. |
Energy use and running costs
The Cisco CP-8851 draws 3.3 W in standby — the idle state it spends nearly all of its time in, connected to the network but not on a call. Plugged in 24 hours a day, 365 days a year, that is about 28.9 kWh per year, or $4.91 at $0.17 per kWh.
As a PoE Class 3 device, the phone can draw up to 12.95 W at peak (for example with the backlight on or during a call on speaker), though typical idle draw is the 3.3 W measured above. The switch's own losses come on top of this.
Off-mode power is 0.21 W.
Talk time adds very little energy compared with standby, because calls are short relative to the hours the phone sits idle.
Frequently asked questions
What kind of phone is the Cisco CP-8851?
It is a VoIP desk phone, listed in ENERGY STAR's corded configuration.
How much electricity does the Cisco CP-8851 use?
It draws 3.3 W in standby, about 28.9 kWh per year — roughly $4.91 at $0.17/kWh if left on continuously.
Can the Cisco CP-8851 be powered over Ethernet?
Yes. It is a PoE Class 3 device, so an IEEE 802.3af/at switch or injector can power it over the network cable.
Will the Cisco CP-8851 work during a power outage?
Only if the network equipment and PoE switch are on backup power and your phone service is up. Keep another way to call 911.
How many Ethernet ports does the Cisco CP-8851 have?
It has 4 Ethernet ports (2 Gigabit). The second port lets a computer share the phone's network connection.
Sources and disclaimer
Specifications on this page come from the ENERGY STAR certified telephones dataset published by the U.S. Environmental Protection Agency (record certified May 22, 2014), retrieved October 7, 2026.
The setup, operating, maintenance and troubleshooting guidance is written by ManualHarbor's editors for this specific product type and configuration. It is an independent guide — not a reproduction of the manufacturer's manual — and ManualHarbor is not affiliated with Cisco. Always follow the safety instructions, installation requirements and warranty terms supplied with your unit, and contact Cisco or an authorized service provider for warranty repairs and replacement parts.
Running-cost figures use assumed U.S. average utility prices and are estimates for comparison only. See our editorial methodology for details.
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