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") ![](/images/getstarted-writetous-navbar-int_ZY7MOf.webp) ### Write to us Fill out our request form. Our experts will respond as soon as possible. [Write to us](/en/request-information "Write to us") ### Calling Line Identification Restriction (CLIR) [How can I withhold my phone number when I call someone?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/clir#section-3) [Does CLIR anonymize my phone calls?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/clir#section-4) [Is my number always hidden?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/clir#section-5) [Can I use CLIR with VoIP?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/clir#section-6) **What is a withheld number?** When a number is withheld, there is no caller ID, which means the recipient cannot see who is calling. A number can be withheld during digital, analogue or [voice-over-IP (VoIP)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) telephone calls. The process of withholding a number is also called Calling Line Identification Restriction (CLIR). The recipient of a withheld number call usually sees a message such as “unknown” or “withheld number” in their phone display instead of a [phone number](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/national-international-phone-numbers). ### CLIR ![](https://a.storyblok.com/f/290788/3586db7412/clir.jpg) ### How can I withhold my phone number when I call someone? Normally, you can activate CLIR for individual calls, or you can install the functionality permanently. For example, landline customers can prefix their dialed number with special digits to make their number unrecognizable to the recipient. The number sequence required for this differs from provider to provider. ### Does CLIR anonymize my phone calls? CLIR allows callers to remain anonymous to a certain extent. However, it is important to mention that CLIR only causes the phone number not to be visible to the recipient. The phone system or the phone provider can recognize your phone number and identify you. ### Is my number always hidden? In most cases. However, there are exceptions, for example in the case of emergency call centers. The fire department, police or emergency services always see the caller's number - even in cases where CLIR is activated. This ensures that help can be provided at any time and as quickly as possible as soon as you dial 911. Another reason is to protect against misuse of such emergency numbers. In addition, there are some legal requirements that say that certain people are excluded from CLIR, for example when it comes to telephone solicitation. ### Can I use CLIR with VoIP? In the case of the [NFON Cloud Telephone System](/en/products/cloudya), VoIP users can benefit from all the advantages and functionalities that traditional telephony has to offer. This includes call number withholding according to your personal preferences. ### Related Articles ### Voice over Internet Protocol (VoIP) PSTN stands for Public Switched Telephone Network and is generally known as the traditional phone network. It has been connecting people since the late 19th century, at first over copper telephone lines and later using fibre optic cables, satellites, and undersea cables. All of these are interconnected by switching centres which establish a dedicated connection – or circuit – between callers. Voice signals can then travel over this circuit. When the caller speaks into his handset, the caller’s phone translates the sound waves into electrical signals. These are transmitted over the PSTN (converted to optical signals for part of the way, so they can travel via fibre optic cables), and the phone at the other end of the line then translates the received signals back into sound. The arrival of **Voice over IP (VoIP)** technology – essentially, transmitting voice calls over the Internet rather than the PSTN – challenged traditional telephony and fundamentally changed how people communicate. 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