Consumers are so infuriated with robocalls that when we called for volunteers to try out four widely used robocall blocker devices plus Nomorobo’s call-blocking technology, more than 130 people responded. We asked each person to install the call blocker that we sent them, monitor the number of robocalls that got through for four days, then disengage it and compare the results. We also asked them to describe how easy it was to set up the robocall blocker.

Some of the devices let you build a “whitelist”; that is, consumers must manually program the phone to recognize and accept a certain number of known “safe” numbers. Some offer a blacklist, meaning that the software is already preloaded with thousands of spam numbers, which are automatically blocked from coming through. Some offer both.

With the exception of Nomorobo, all of the robocall blocker devices could be installed on a landline or a VoIP phone ( “Internet phone”) with caller ID; Nomorobo currently is available only for VoIP phones. (We did not try out call-blocking apps for smartphones.) The prices listed are what is being charged at Amazon.com, our purchasing source—not the manufacturers’ suggested retail price. The Sentry model we tested has since been replaced by the Sentry 2, which makes that robocall blocker easier to set up and add numbers to the whitelist.

Read our special report, "Rage Against Robocalls." And tell us about your experience with a robocall blocker or sound off about robocalls by adding a comment below.

Digitone Call Blocker Plus: $110. Blacklist/whitelist.

Nine of the 24 testers found the setup instructions for this robocall blocker confusing. But consumers appreciated that the device operates in silence: “A flashing red light identifies a successful block. I could see incoming robocall attempts, but the phone did not ring,” wrote one tester. Eighteen out of 24 respondents said they would buy the device.

The buzz: Buy

Nomorobo: Free. Blacklist/whitelist.

Nomorobo intercepts all calls after the first ring, compares the number to its vast list of robocall originators, and decides whether to let the call go through. Recipients hear the first ring; if the call is legitimate, the phone rings normally. “Only blocked one call that I wanted,” raved one tester. Once he added the number to the wanted-call list, “they got through the next time. I seriously could not be happier.” Among 40 testers, 25 gave Nomorobo top marks on a scale of 1 to 5, and nine rated it 4.5 or 4.

The buzz: A winner

HQTelecom.com Landline Call Blocker: $59. Blacklist.

One tester wrote, “The device is not ‘proactive,’ i.e., it does not block robocalls until I press the block button.” (He had to answer the call.) “Then further calls from that number will be blocked.” Other testers complained that numbers they had manually blocked continued to get through. Among the 13 respondents, six said they would buy this robocall blocker; seven said they wouldn’t.

The buzz: Mixed

Sentry Dual Mode Call Blocker: $59 (Sentry 2). Blacklist/whitelist.

Respondents thought the Sentry did an excellent job of thwarting unwanted calls; receiving wanted calls was more problematic. “There is no option to manually add numbers to the accept list,” a tester said, leading him to worry about missing infrequent but important messages, such as prescription refills or occasional calls from old friends. Legitimate callers can get through the Sentry’s block by listening to a recorded message and pressing 0 to be connected. But the recording is made in a British accent, leading some callers to assume that they’d reached a wrong number unless they had been warned what to expect. Twenty-seven respondents voted yes; 28 voted no.

The buzz: Mixed

CPR Call Blocker Protect: $45. Whitelist.

The whitelist-only functionality protects people who are easily taken advantage of but restricts the device’s effectiveness. A typical comment: “It blocked every call that came in, unless I programmed each number that I wanted to receive. This was a huge setback, as there is no possible way for me to program every caller I need to answer.” Eight out of 10 respondents said they would not buy this robocall blocker.

The buzz: Bummer

This article also appeared in the September 2015 issue of Consumer Reports magazine.

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