The latest in a long line of iPhone hacks that have promised to increase cellular download speeds on AT&T-, Verizon-, Sprint-, and T-Mobile-powered handsets does more than just boost cell signal. Instead, the hacked carrier update, which is available for T-Mobile users only, can also activate the iPhone 5-only HD Voice feature on iPhone 4 and iPhone 4S handsets.

As a reminder, HD Voice technology dramatically enhances clarity during phone calls. Though AT&T promised back in April that support for the feature would launch “later this year,” T-Mobile iPhone 5 handsets included support for HD Voice from launch. In all cases, the feature is limited to Apple’s sixth-generation smartphone, along with models manufactured by the likes of Samsung, HTC, LG, and others.

After installing the aforementioned hack, however, HD Voice can be enabled on iPhone 4 and iPhone 4S handsets. In addition, the T-Mobile carrier update also hopes to enhance cellular download speeds, and adds a 4G toggle button that allows users “to switch between EDGE and HSPA+ for stronger signal when HSPA+ is not available, and to conserve battery.”

As explained at iTweakiOS:

iPhone 5: This hack enables Release 9 DC-HSPA+ on the iPhone 5 as well as IPv6 capabilities. Though T-Mobile has not yet upgraded their network to support these speeds, the device itself becomes more open to an increased amount of bandwidth thus allowing for better throughput of data and data connection. This hack also includes a fix to any and all iPhone 5 users who have experienced degraded/weakened signal over the past couple of months on the PCS and/or AWS bands, though a lot of this could be due to the eminent release of LTE in many upcoming markets as this is normal. The issue was due in part to the carrier update. It set the band preference for HSPA+, even if your iPhone 5 doesn’t support it, to AWS 1700MHz which can cause conflictions and problems for those devices that don’t support that band. The setting has been set back to the default “auto” setting and should help the 4G signal perform a bit better, whether your iPhone 5 is PCS only or dual PCS/AWS. For those with an iPhone 5 that supports AWS, this hack will also help you as well as it will connect to the strongest tower (PCS and AWS) rather than the preferred type of tower.

iPhone 5 (A1429): This hack does the same as mentioned above, but since you guys are left out of the LTE loop, we figured we would give you guys some functionality back. Rather than having the LTE Toggle, you guys now have the 4G Toggle to switch between EDGE and HSPA+ for stronger signal when HSPA+ is not available and to conserve battery, especially if you are on iOS 7 and experiencing any battery draining issues.

iPhone 4/iPhone 4S: This hack enables HD Voice like what the iPhone 5 users have and gives 4S users a slight speed boost for HSPA+. It also gives some network optimiation settings that may benefit you as well. This also includes IPv6 capabilities as well.

What’s more, the above method is compatible with the recently released iOS 7 beta 1.

For more information, including instructions and download links, be sure to visit iTweakiOS. Do remember, however, to proceed with caution when applying hacked carrier updates to an iPhone handset, and back up your smartphone in iTunes first!