Which cellphone company is best at blocking robocalls?

If you want to stop the spam and scam calls to your cellphone, the carrier you choose matters. That's the takeaway from a recent study of four major carriers — AT&T, Sprint, Verizon and T-Mobile — which found that most carriers are still poor at identifying spam and scam calls, even when consumers pay for the service.

Cellphone carriers have only had the opportunity to identify and block spam and scam calls for four months. In November, after being flooded with complaints about incessant robocalls, the Federal Communications Commission changed longstanding rules that required carriers to connect all calls. The agency now allows carriers to identify and block calls coming from known scam numbers, as well as those that can be identified as spam or coming from spoofed or non-existent numbers.

Most of the major cell carriers now offer caller ID and blocking applications, several of which require the consumer to pay a monthly fee. How effective are these services? Shuffle Ventures, which sells its own telephone privacy applications, decided to test the services by commissioning Lionbridge, which has an app-testing service. Lionbridge made 2,000 calls on each network, evenly split between Androids and iPhones -- 8,000 calls in total. 

The findings: 

The one carrier that offered free call blocking — AT&T —was the least effective overall. AT&T correctly identified only 14 percent of the scam and fraud-related calls; half of the spam and telemarketing calls and did not provide accurate caller identification. 

T-Mobile, which offers scam blocking for free, was most effective at identifying scam and fraud calls, earning a 90 percent efficiency rating in the study. However, it was slightly less effective than AT&T at spotting the spam and telemarketing calls, ferreting out just 47 percent of those. T-Mobile's caller identification service, which costs $4 per month, correctly identified 78 percent of the callers, the study said. 

T-Mobile was ranked best overall, earning a 91 percent efficiency rating in this survey, versus a 25 percent rating for AT&T.

Verizon, which charges $3 per month for caller ID, correctly identified 78 percent of the calls, and blocked 35 percent of the spam and telemarketing calls. But it was not at all effective at identifying scam and fraud calls, earning Verizon an overall rating of 65 percent.

Sprint, which also charges $3 monthly for premium caller ID services, correctly identified 73 percent of calls, but was not effective at pointing out the scam and spam calls. Overall, Sprint earned a 51 percent efficiency rating, according to the Lionbridge study. 

These findings appear to bolster the case made by consumer groups, many of which are pressing the FCC to do more to block spoofed robocalls. A coalition of such groups noted in comments on Friday that the agency's November ruling is not mandatory, and could be more effective.

"Bad actors are continuing to find ways around the rules to prevent fraudulent robocalls and take advantage of consumers but there is more that can be done to protect consumers," said Maureen Mahoney, policy analyst at Consumers Union. "The FCC should ensure that consumers can control the calls they receive by requiring that phone companies provide blocking technology free of charge to consumers."