One of the simplest ways to manage unwanted calls and telemarketing spam is to block your phone from ringing, routing the incoming call to voicemail. Callers will have to leave a message, and many telemarketers won't bother. But some callers, especially the robocalls, will, flooding your mailbox with sketchy debt relief or vacation club offers.

Unfortunately, preventing spam callers from leaving a voicemail on your cell phone can be tricky. But there are a few options that really do work if you’re on the right carrier, the right phone or willing to change the way you manage your phone calls.

Blocking Calls on iPhones

On iPhones running iOS 8 or higher, go the Recent Calls tab in the Phone app, tap the “i” icon at the right of the number that you want to block. If the number is in your Contacts list, open that contact’s page in the Contacts app. From the contact page for the caller or contact, scroll to the bottom of the page and tap Block This Caller.

But this method has two big drawbacks. First, someone has to call you in order to block the number. And since spammers rarely use the same number, or use the “neighborhood spoofing” technique, this won’t block the majority of spam calls we get. Second, those blocked calls still go to voicemail, though you won't get a notification. So, this isn’t really much help.

Blocking Calls on Android Phones

Android has built-in call blocking, similar to iOS. Just tap a number in your call log and hit Block/report spam. And like Apple, this method has the two same drawbacks.

Android also has the option in the Phone app settings to automatically identify and block suspected spam calls so they don’t ring your device. It should be on by default, but you can check by going to Settings in your Phone app, then Caller ID & spam, and make sure both the Caller ID & spam and Filter spam calls switches are toggled on. This feature still sends those calls to voicemail, however.

One exciting development is a new feature for Pixel phones that will allow Google Assistant to answer a suspected spam call and transcribe the conversation in real time, so you can decide if you want to answer the call or hang up. And since Google Assistant already picked up the call, hanging up won’t send it to voicemail. Initially, you still have to read the transcript and take action, which may be more of a pain than simply answering the call yourself. Hopefully, Google will add the option to auto-hangup if it can tell it’s a robocaller.

Spam Blocking Apps for Android & iOS

There are dozens of apps in the Apple App Store and Google Play that are labeled as call blockers. But you have to be very careful about which you install – you’re giving them full access to your phone, texts, contacts and more, meaning there’s a lot of potential for misuse. And each of those apps will only perform as well as the technology (and the blacklist) running it. Also, while these apps will all block calls, because of restrictions placed on apps by the Android and iOS operating systems, none can stop those calls from going to voicemail.

I strongly recommend going with one of the brand name choices, such as Hiya or Truecaller. Personally, I prefer Hiya (which whitelabels its technology to Samsung, AT&T and others), has an extensive community-driven blacklist, allows you to block neighborhood spoofing calls, and even provides caller ID features that you would usually have to pay for with your carrier. Better yet, Hiya is free for Android and iOS and gets excellent reviews on both platforms. Hiya also offers a $2.99 per month premium version for iOS with more advanced caller ID, name lookups, and more frequent spam updates (3x/day vs 1x/day on the free version).

Blocking via Google Voice

Google Voice provides another way to block pesky spam calls and prevent them from going to voicemail. The trick is you need to switch to Google Voice as your main number and stop giving out your old carrier number. With Voice, you can block known spam calls in three ways: by sending calls to voicemail, by treating the call as spam (letting the caller leave voice mail but tagged as spam) or by call blocking (in which case the caller will hear a “Number not in service” message and will not be able to leave voice mail).

The big drawback here is that your Google Voice number now becomes your main number, which you forward to the number from your carrier, and that takes some work to transition. And there’s still no guarantee that spam callers won’t call your carrier number directly, either because it’s already out there (learn how telemarketers get your number) or simply because the robodialers are going through every number combination.

Carrier-level blocking

Truly effective call blocking and voicemail prevention should be at the carrier level. It’s the carriers who have the technical capability to identify call origination sources and create services that prevent spam and blocked calls from going to voicemail (since they’re the ones that control the voicemail service). And all the carriers have some level of blocking on their networks to weed out the most obvious and egregious spammers. Yet, while the FCC has been working with carriers to develop more robust tools, options are still limited, and carriers have been very slow at implementing technology to make the robocall problem go away entirely.

Call blocking on AT&T

Of the four major carriers, AT&T has the most effective tools for blocking spam calls and it’s the only one that actually lets you stop these calls from going to voicemail. The drawback is you have to pay $3.99 a month for the privilege.

At a basic level, AT&T postpaid customers with HD Voice Android and iOS phones can activate AT&T’s free Call Protect service on their accounts. Call Protect will block known spam calls entirely, preventing them from leaving a voicemail. And with the companion Android or iOS app you can also block specific numbers.

If you want more features, you can upgrade to Call Protect Plus for a pricey $3.99 a month. Call Protect Plus users can block categories of calls, including: private callers, debt collectors, telemarketers, surveys, and general spam. You can choose to have these calls go to voicemail or be blocked entirely. Unfortunately, there is no option to block local number spoofing.

Call blocking on Sprint

Sprint has the weakest offering of the four main carriers. Their Premium Caller ID service provides caller information on incoming calls, including spam warnings, but that’s it. There’s no option to block known spam calls or prevent them from going to voicemail. And, you have to pay $2.99 a month for this basically useless feature.

Call blocking on T-Mobile

T-Mobile sits in the middle when it comes to spam call protections. For postpaid customers, T-Mobile’s free Scam ID service is automatically activated and will notify you of potential spam calls. But what you really want is their Scam Block feature, which will block these calls from ever reaching your device and prevent the caller from leaving a voicemail. Scam Block is also free but requires you to manually activate it, which you can do simply by dialing #662# from your phone (you can also disable it by dialing #632# if it is blocking any valid calls). There is no option to block categories of calls or neighborhood spoofing. Though, for $4 a month, you can get the not-very-useful Name ID service with more advanced caller ID and caller category info.

Call blocking on Verizon

Verizon’s Caller Name ID service will identify likely spam callers and give you the option of automatically blocking them based on risk level: High Risk, Medium Risk or All Risk Levels. However, those blocked calls will still go to voicemail and Caller ID requires a $2.99 per month subscription fee – in essence, you’re getting the same level of blocking that Android provides for free, but Verizon is charging you for it. Er, no thanks.

While all of the options above will help you block calls and, in some cases, stop spammers from leaving a voicemail, the best way to prevent these calls in the first place is to learn how telemarketers get your cell phone number — and then don’t let them have it. Prevention is better than cure, after all. And we have advice for blocking spam on your landline, too.

[Image credit: scam call via Shutterstock]

Updated on 10/15/2018 with new recommendations