Microsoft revealed on Tuesday that U.S. network operator Sprint and Spanish operator Telefonica are both still testing the company’s upcoming Windows Phone 7.5 “Mango” update.

Microsoft is widely expected to launch Windows Phone 7.5 “Mango” on a variety of existing devices on Tuesday but Sprint and Telefonica devices may have face a delay. The software giant updated its “Where’s my phone update?” pages on Tuesday to indicate that all network operators worldwide, apart from Sprint and Telefonica, are in the scheduling phase for Windows Phone 7.5. Microsoft has promised that existing devices will receive the update in the next week or two.

Telefonica and Sprint are both marked as “testing”, a process controlled by the network operator. Microsoft explains that carriers perform mobile operator network and quality tests. “These tests can vary by mobile operator, software update, and device,” says Microsoft. Operators can also include device-specific improvements or customizations for their particular network. Microsoft originally delivered its pre-NoDo update on February 21, followed by the full NoDo update on March 22. Telefonica and others have stalled on carrier testing to allow the update to be pushed to their devices.

It’s not immediately clear when Sprint and Telefonica Windows Phone 7.5 updates will be available for customers on those particular carriers. Sprint’s own support pages indicate that the company could be preparing to release the update on September 27.  AT&T is expected to start distributing Windows Phone 7.5 “Mango” updates to most of its devices on September 27. A leaked memo revealed that the company will offer Windows Phone 7.5 to HTC Surround, LG Quantum and Samsung Focus 1.3 devices initially. AT&T will be rolling out Windows Phone 7.5 “Mango” to the HTC HD7S in early to mid-October and the revision 1.4 version of the Samsung Focus should receive the update by the end of October.

Microsoft revealed last week that Windows Phone 7 users should expect to see the Windows Phone 7.5 “Mango” update within the next two weeks. Microsoft warned users not to install leaked copies of Windows Phone 7.5. “Every Windows Phone will also receive software from the handset manufacturer,” explained Microsoft’s Eric Hautala. “Since your phone requires the proper firmware to function as designed, my advice is simple: steer clear of bootleg updates and homebrew tools.” Copies of leaked Windows Phone 7.5 RTM builds have been made available for various devices over the past several weeks.