According to a purported leaked T-Mobile document obtained by TmoNews, the carrier will offer an unlimited mobile-to-mobile calling add-on for $10 a month starting April 4th. “Beginning April 4th, T-Mobile will offer Unlimited Any Mobile, a $10 per month subscriber-level feature that includes unlimited calling to any domestic mobile number, including Puerto Rico, regardless of carrier,” the document reads. The carrier claims that 80% of calls made on its network are to mobile numbers, making such an add-on appealing to subscribers looking to save money. The feature will be available as an add-on for existing customers, and it will not require a new contract. Unlimited out-of-network mobile calling is nothing new; Sprint bundles a similar offering with its unlimited everything plans and AT&T has an add-on as well. A screen capture of the purported T-Mobile document follows below.