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A key telephone system (KTS) lets multiple telephone lines or services appear on a single telephone. The Federal Communications Commission (FCC) also calls it a “Key System,” “Com Key System,” or “KSU.”
What is a key telephone system?
The FCC defines a Key System as a phone system that permits more than one telephone line, PBX extension, private line, or intercom line to appear on one telephone. The defining feature is how multiple lines or services are presented to the user—not a promise that every system supports every listed line type.
In practical terms, a user can have several line appearances available on a telephone rather than needing a separate telephone for each line. The exact arrangement depends on the system and its configuration.
Source: Federal Communications Commission, FCC 13-100 glossary, “Key System (KSU)”.
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AT&T 3 Handsets Cordless Phone with Answering System — BL108-3
- SMART CALL BLOCKER — STOPS ROBOCALLS BEFORE THEY RING — Automatically filters out robocalls on the first call — no setup required. One-touch blacklist any number on the spot. Block directory holds up to 1,000 entries
- BLUETOOTH CONNECT TO CELL — Pair your smartphone and make or receive mobile calls through any of the three handsets. Routes calls through the base to eliminate cellular dead zones at home
- 3 HANDSETS, ONE PHONE JACK — 2.4" COLOR DISPLAY — All three handsets connect from a single base. Extra-large color LCD, lighted keypad, and status icons for voicemail, battery, and missed calls on every handset
- DIGITAL ANSWERING MACHINE + CALLER ID ANNOUNCE — Records up to 22 minutes of messages. The handset speaks the caller's name aloud — screen calls from any room without touching the phone
- AUDIO ASSIST & LONG-RANGE DECT 6.0 — Audio Assist boosts call volume and reduces background noise mid-call. DECT 6.0 with advanced antenna delivers interference-free sound up to 1,000 ft
What do KTS, KSU, and Com Key System mean?
They are alternate terms associated with the same system category in the FCC glossary: “Key System,” “Com Key System,” and “KSU.” The glossary’s definition describes the system’s ability to present multiple lines or services on a telephone.
Is a key telephone system the same as a PBX?
No. Under U.S. law, both key telephone systems and private branch exchange (PBX) systems are types of multi-line telephone system (MLTS). The law also includes hybrid systems and other arrangements in that broader category. MLTS is the umbrella term; it is not another name for KTS.
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VTech IS8251-3 Business Grade 3-Handset Expandable Cordless Phone for Home Office, 5" Color Display, Programmable Short Cut Keys, Smart Call Blocking, Answering System,Bluetooth Connect to Cell
- [Business-grade Cordless Phone built for ease of use at home] VTech IS8251 cordless brings excellent clarity and range, ensuring highest quality in voice communications for business or private calls. Featuring 5" color base display, large icons, soft keys for seamless call management. Programmable short-cuts for quick access to phone functions that are important to you.
- [Enhance efficiency] One-touch access to important contacts; One-touch to call back the caller from listed message or call history; Timer function to help you split your time doing tasks more efficiently.
- [Never missed a call] An Virtual 2-line system manages calls from landline and cell-line simultaneously. Pair up to 2 cell phones or one cell phone and one Bluetooth headset for use with landline calls.
- [Advanced Digital Answering System] The 5" color base LCD displays the caller information of the message received, allows you to choose who to listen first from the message list displayed. Extra long 40 mins recording time ensuring all incoming messages captured.
- [Smart Call Blocker] Say goodbye to unwanted calls. Robocalls on your landline are automatically blocked from ever ringing through—even the first time.
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Panasonic Cordless Phone with Answering Machine, Advanced Call Block, Bilingual Caller ID and High-Contrast Display, Expandable System with 3 Handset Cordless Telephones - KX-TGD833M (Metallic Black)
- Advanced Call Blocking: Automated Call Block pre-blocks robocalls; Telemarketing Call Block lets you to block announced callers; block 1,000 more with 1-touch Call Block Button on the cordless phones
- 2-Way Recording: Never miss important details again--record your telephone conversations to replay later; the announcement of “Start Recording” on this wireless home phone effectively deters suspicious callers
- One-Ring Scam Alert: This house phone protects you from one-ring scams by displaying a confirmation screen when returning a one-ring call
- Panasonic phones continue to connect families and friends all over the globe, offering reliable connection and intuitive design—proudly built on a legacy of Japanese innovation and craftsmanship
- Easy to Find and Use: Illuminated keypad with large characters is findable, even in the dark; speed dial gives you 1-touch access to your most-dialed numbers; wall-mountable design keeps home phones reliably close
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AT&T EL52113 Cordless Phone with Answering System & Extra-large Backlit Keys Light champagne
- Extra-Large Backlit Keys: Oversized, illuminated buttons and big font for easy dialing in low-light conditions and enhanced visibility for seniors
- Digital Answering System: Up to 14 minutes recording time with remote access, call screening, and message retrieval from the handset
- Caller ID/Call Waiting: Stores 50 calls with name and number display to help screen unwanted calls easily
- DECT 6.0 Clear & Secure: Interference-free calls with long range and private digital transmission for crystal-clear conversations
- Full-Duplex Speakerphone: Allows both ends to speak and be heard simultaneously for natural, hands-free conversations
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AT&T CD4930 Black Corded Phone with Digital Answering System, Caller ID, Extra-Large Tilt Backlit Display & Big Buttons
- 25-Minute Digital Answering System — Never miss a message again. Record up to 25 minutes of incoming calls, outgoing announcements, and memos with easy instant playback, selective save/delete, and the ability to skip or repeat messages right from the handset or base
- Extra-Large Tiltable Backlit Display — See Caller ID names, numbers, time, and date at a glance — even Call Waiting. The oversized LCD tilts for the perfect viewing angle, reducing strain for seniors and anyone with vision needs
- Oversized Big Buttons — Large, high-contrast keys are easy to read and press — ideal for elderly users, arthritis, or low vision. Simple, frustration-free dialing every time
- Hands-Free Speakerphone — Talk and listen comfortably without holding the handset. Perfect for multitasking, note-taking, or including the whole family in the conversation
- Reliable Corded Design with Audio Assist — Works even during power outages. Audio Assist temporarily boosts volume and clarity for better hearing, plus an extra-loud ringer with visual flashing indicator
Source: 47 U.S.C. § 1471.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What does the definition not tell you?
- It does not specify a universal feature set. A KTS may present telephone lines, PBX extensions, private lines, or intercom lines; the definition does not say every installation includes all of them.
- It does not establish compatibility. The category name alone cannot tell you whether a particular modern handset will work with a specific legacy system. Compatibility depends on the equipment and installation.
- It does not define capacity, cost, or complexity. The cited sources classify the system and describe its line appearances, but do not provide a general comparison on those points.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
