A bill slowly working its way through Congress would once again make it legal for consumers to unlock their cellular phones so that they can be used on other carriers' networks.

While lawmakers dither, a "voluntary" agreement the carriers signed under pressure from the Federal Communications Commission is paying off for some customers, but not all.

The guarantee signed in December should force the carriers to unlock phones, albeit only when a consumer has paid off his or her contract, and in many cases not until early next year.

Further Reading Unlocking new cell phones to become illegal on Saturday

The bill moving through Congress is stronger because it would let consumers unlock their phones at any time, even before they've paid off their contracts. That wouldn't free consumers from having to pay their bills, but there are reasons to want an unlocked phone even while you're paying a monthly fee to your primary provider. For example, a trip overseas could be a good time to swap out a phone's SIM card and connect to a local network in order to avoid a US carrier's international data and voice charges.

"Carriers could voluntarily unlock your phone before this law because they have a license to do so from their relationship with handset and software makers," Chris Lewis, vice president of government affairs for consumer advocacy group Public Knowledge, told Ars. "With this law, individuals will no longer have to rely on carriers to do the unlocking."

Since Congress hasn't taken final action, all we have now is the carriers' voluntary agreement. The carriers don't have to follow all the provisions of this agreement until February 11, 2015, so they don't have to unlock all out-of-contract devices until then. But some carriers are already doing so.

There are six provisions: Disclosure of an unlocking policy on the carrier's website; unlocking postpaid phones after a contract is paid off; unlocking prepaid phones one year after activation; providing notice to customers when their devices are eligible for unlocking or automatically unlocking them remotely when eligible; unlocking phones for deployed military personnel; and guaranteeing a two-day response time to unlock requests. (The response time isn't an obligation to unlock a phone—carriers can comply by explaining to customers why their devices do not qualify for unlocking.)

Carriers agreed to implement at least three of the six obligations by May 11, 2014, and the rest by next February. It's thus possible to comply with the agreement today without having to unlock the phones of all consumers with off-contract devices. While we're talking primarily about phones, the agreement covers cellular devices in general and thus should apply to other cellular-enabled devices such as tablets.

T-Mobile, US Cellular move faster than bigger rivals

We asked AT&T, Verizon Wireless, Sprint, T-Mobile, and US Cellular this week to update us on their progress.

T-Mobile and US Cellular are leading the way by meeting five of the six obligations already. All that remains for both T-Mobile and US Cellular is the sixth, providing notice to customers when they're eligible for unlocking.

Sprint says it has met four obligations: disclosure, unlocking postpaid phones, two-day response times, and unlocking phones for deployed personnel. That leaves the requirements for unlocking prepaid phones and giving customers notice when their devices are eligible for unlocking. Sprint's policy says it doesn't yet unlock prepaid phones but that it is "working to create new policies and procedures in order to unlock prepaid devices."

AT&T told Ars it has met three obligations: disclosure, unlocking prepaid phones, and unlocking phones for deployed personnel. You might be able to get postpaid phones unlocked by AT&T as well, as the company's policy says it will unlock the phones upon request if the customer has met a contract's requirements.

A Verizon Wireless spokesperson told Ars that the company has complied with three standards: disclosure, the response time standard, and unlocking devices for deployed military personnel. That leaves three obligations that won't be fully complied with until next year: the actual unlocking of prepaid and postpaid phones and providing notice to customers when their devices are eligible for unlocking.

The situation isn't so bad on Verizon, though, because its newer phones generally ship unlocked out of the box. Verizon's device unlocking policy says, "We do not lock most phones or tablets that are activated with our postpay service, either during or after the term of your service contract or Edge installment sales agreement. We do not lock our 4G LTE devices, and no code is needed to program them for use with another carrier. We do not lock our Prepaid devices, other than our Phone-in-the-Box Prepaid phones, and the simple code to program such Prepaid devices for use with another carrier is either '000000' or '123456.'"

There are some exceptions to that policy: Verizon still locks the SIM cards in its "3G Global Ready" phones. "By the end of January, 2015, we will unlock those SIM cards at your request once you have fulfilled the term of your service contract or Edge installment sales agreement and your balance is current," Verizon's policy says.

It's a bit of a crapshoot with older devices. One Verizon customer with an iPhone 4S told Ars that he hasn't been able to get his device unlocked, with Verizon telling him that option won't be available until 2015.

Cellcom, Cellular One, and Illinois Valley Cellular also signed on to the phone unlocking agreement, which was negotiated between the CTIA Wireless Association and the FCC. FCC Chairman Tom Wheeler had told the carriers he would issue regulations if they did not comply with his terms.

Why changing the law is still important

Even though carriers often do unlock off-contract phones, a new law legalizing phone unlocking would still bring benefits to consumers.

"All of them have agreed to unlock phones for consumers, but that is a statement of their own policy, it doesn't have the force of law, and they can revise it whenever they like," Derek Khanna, a Yale Law fellow who helped lead a campaign that led to the White House supporting legalized unlocking more than a year ago, told Ars this week. "It has no legal binding effect, allowing them to choose to implement it as they see fit, and we already know that in the past when carriers have said they allow consumers to unlock, they didn't really mean it."

Under the current scheme, unlocking phones to run on other networks violates the Digital Millennium Copyright Act (DMCA) prohibition against "circumventing" copy protection schemes.

The latest legislation is a compromise bill proposed by Sen. Patrick Leahy (D-VT) and Sen. Chuck Grassley (R-IA). It is intended to eliminate the prohibition and let consumers unlock phones "when their contract expires," according to a press release from Leahy. The bill text itself doesn't make any distinction between off-contract and contract phones, however.

"The Leahy legislation would allow consumers to unlock their phones whenever they please," Khanna told Ars. "This may or may not be a contractual violation, but under Leahy's legislation a consumer unlocking will not be a federal legal matter under the DMCA. Let's say a consumer pays off their contract but then passes on the phone to a family member. If the carrier chooses not to allow that family member to unlock, as they have done in the past, then one could unlock their device without legal liability. Similarly, if a carrier denies the consumer the ability to unlock because he/she doesn't have a receipt or if they deny a service member the ability to unlock when deployed, both of which they have done in the past, then one could unlock their own without facing any legal liability—this is a good thing for the consumer and the market."

An earlier version of the legislation that passed in the House would have banned "bulk" unlocking, making it impossible to acquire a large number of phones, unlock them, and then resell them. The new Senate version of the bill would not ban bulk unlocking, but it's not clear when or if it will be passed.

It is still legal to unlock phones purchased before Saturday, January 26, which is when a DMCA exemption that previously allowed phone unlocking was eliminated.