The fastest way to block unknown numbers
Most phones let you block all calls from numbers that don't show a caller ID. On iPhone, go to Settings > Phone > Silence Unknown Callers, and toggle it on. Unknown calls will go straight to voicemail without ringing. On Android, the method varies by manufacturer: Samsung phones have Settings > Call > Call Rejection > Reject Unknown Callers, while Google Pixel phones use Phone app > Settings > Spam and Call Screen > Filter Unknown Calls.
These built-in tools work when ready and don't require you to install anything. They block calls from numbers that transmit no caller ID information — which includes many spam and scam calls, but also some legitimate calls from hospitals, banks, and government offices that use systems that don't send ID data.
If you want to block specific numbers instead of all unknown callers, you can add them to your phone's block list one at a time. On iPhone, open the recent call, tap the info icon, and select Block This Caller. On Android, open the Phone app, find the number in your call history, tap the three dots, and choose Block Number.
Key Takeaways
- iPhone's Silence Unknown Callers and Android's Filter Unknown Calls send all unidentified calls to voicemail without ringing your phone.
- Blocking unknown calls will also block some legitimate calls from hospitals, banks, and government agencies that don't transmit caller ID.
- You can block individual numbers through your phone's call history or contacts app without blocking all unknown callers.
- Third-party call-blocking apps like Nomorobo and RoboKiller offer more detailed filtering but require you to grant them access to your call data.
What happens to blocked calls
When you block a number on your phone, the call doesn't ring through to you. On iPhone with Silence Unknown Callers on, the call goes directly to voicemail. The caller hears a message saying the call couldn't be completed, or they may be able to leave a voicemail that appears in your voicemail inbox.
On Android, the behavior depends on your phone model and settings. Some phones send blocked calls to voicemail; others may send a text message back to the caller saying the number is not available. You can usually see a log of blocked calls in your Phone app under a "Blocked" or "Spam" section, so you can review them later if needed.
The key difference between blocking and silencing is that silencing (like iPhone's Silence Unknown Callers) stops the ringing but still logs the call, while blocking an individual number may prevent the call from reaching you at all. Neither method deletes the caller's number from existence — they can still try to reach you, but you won't be interrupted.
Using third-party call-blocking apps
Apps like Nomorobo, RoboKiller, and Truecaller offer more control over which calls get through. These apps can identify spam patterns, block numbers based on keywords in voicemail transcripts, and let you create custom rules (for example, "block all calls from numbers I've never saved"). Some apps also show you a spam score for incoming calls before you answer.
The trade-off is that these apps need permission to access your call history, contacts, and sometimes your location data. They use this information to improve their spam detection, but it means you're sharing phone data with a private company. Read the app's privacy policy before installing to understand what data they collect and how long they keep it.
Most third-party apps offer a free version with basic blocking, and a paid version (usually $3 to $10 per month) with advanced features like voicemail transcription and custom filtering. Some carriers like T-Mobile and Verizon also include call-blocking tools in their plans at no extra cost — check your carrier's website to see what's available to you.
The trade-off between blocking unknown calls and missing legitimate ones
When you block all unknown callers, you also block calls from numbers you don't have saved, even if they're legitimate. A hospital calling to confirm an appointment, your bank calling about fraud, or a government office calling about a tax refund — all of these may come from numbers that don't transmit caller ID or use numbers you've never seen before.
If you need to receive calls from unfamiliar numbers, consider blocking only specific numbers instead of all unknown callers. You can add numbers to your block list as they call you, which takes longer but gives you more control. Alternatively, you can turn on Silence Unknown Callers only during certain hours (like evenings or weekends) and turn it off during business hours when you're more likely to receive important calls.
Another option is to check your voicemail regularly. Most blocked calls leave a message, so you can listen to voicemails and call back numbers that matter. This approach requires more effort but avoids the risk of missing something important.
How spam callers get your number
Spam and scam callers obtain phone numbers through data breaches, public directories, random number generation, and purchased lists. When you sign up for online services, your number may be sold to marketing companies or stolen in a breach. Scammers also use software to dial thousands of numbers in sequence, so even if you've never given your number to anyone, you may still receive spam calls.
Blocking unknown numbers reduces the volume of calls you receive, but it doesn't stop your number from being on spam lists. If you want to reduce spam further, you can register your number with the National Do Not Call Registry at donotcall.gov. This doesn't stop all spam — political organizations, charities, and companies you've done business with can still call — but it does reduce calls from telemarketers.
Never give your number to unknown websites or callers who ask for it. If a caller claims to be from your bank or a government agency and asks for personal information, hang up and call the organization directly using a number from your statement or their official website. Legitimate organizations don't ask for sensitive information over the phone.
When to unblock a number
If you blocked a number by mistake or want to give someone a second chance, you can unblock them. On iPhone, go to Settings > Phone > Blocked Contacts and swipe left on the number to unblock it. On Android, open the Phone app, go to Settings > Blocked Numbers, find the number, and tap Unblock.
You might unblock a number if you realize it was a legitimate call (like a doctor's office), if someone you know is calling from a different phone, or if you want to receive calls from a business you've decided to work with. Unblocking is when ready — the next time that number calls, it will ring through normally.
If you unblock a number and then decide you want to block it again, you can do so at any time. There's no limit to how many times you can block and unblock numbers, so you can adjust your settings as your needs change.
Frequently Asked Questions
Will blocking unknown callers stop all spam calls?
No. Blocking unknown callers stops calls from numbers that don't transmit caller ID, but many spam callers use spoofed numbers that appear to be from real businesses or local area codes. You'll still receive calls from these numbers because they show a caller ID. A third-party app with spam detection can catch more of these, but no method blocks all spam.
Can someone tell if I've blocked them?
Not directly. When you block a number, the caller hears a message saying the call couldn't be completed, which is the same message they'd hear if your phone was off or you had no service. They won't receive a notification saying they've been blocked, but if they call repeatedly and always get the same message, they may suspect it.
What's the difference between blocking and Do Not Disturb?
Do Not Disturb silences all calls and notifications, but the calls still come through and appear in your call log. Blocking prevents specific calls from reaching you at all. Do Not Disturb is better if you want temporary quiet; blocking is better if you want to permanently stop calls from certain numbers.
Do I need to pay for call blocking?
No. Your phone's built-in blocking tools (Silence Unknown Callers on iPhone, Filter Unknown Calls on Android) are free. Third-party apps offer free versions with basic features and paid versions with advanced features, but you don't have to pay to block calls.
If I block a number, can they still leave voicemail?
Usually yes. Most blocked calls go to voicemail, and the caller can leave a message. You'll see the voicemail in your inbox but won't be notified of it the way you would be for calls from unblocked numbers. You can listen to it later if you want to.