In Part 4 of my review of Windows Phone 7.5, I explained the availability of three new Windows Phone 7.5 features--Internet sharing, hidden Wi-Fi network support, and Visual Voice Mail--based on a briefing I had had earlier with Microsoft. Confusingly, these features all depend on your wireless carrier supporting them and in many cases, that will only happen first (or ever) on new devices, and not on first generation Windows Phone handsets.

In a support document posted to its Windows Phone web site recently, Microsoft has clarified how these and other new Windows Phone 7.5 features will be made available. So let's take another look, since we can assume this document represents most accurately how these features will roll out around the world.

Availability. First, remember that "the availability of certain Windows Phone 7.5 feature and service experiences varies by country or region." Not coincidentally, I just posted about Andrew Birch's excellent Windows Phone 7.5 feature tables, so go check those out for the latest on which features and services are available in which locales.

Wi-Fi networking. This one is rather amazing: "Your mobile operator or hardware manufacturer can choose not to include support for Wi-Fi networking." What?? In my Windows Phone 7.5 review I note that WP 7.5 now supports "hidden Wi-Fi networks ... but only if your carrier OKs it and makes it available. It will be enabled only on new devices first, but could be back-ported to first gen devices too." That said, I've received emails from readers seeing this feature on some first-gen devices already. For more info, read How to connect to a hidden Wi-Fi network (scroll down).

Bluetooth. Ditto: "Your mobile operator or hardware manufacturer can choose not to include support for Bluetooth."

Internet connection sharing. "Your mobile operator or hardware manufacturer can choose not to include support for Internet connection sharing." In my review, I noted that "Windows Phone 7.5 will indeed support 3G wireless network sharing with up to five devices [with] optional WPA2 wireless security ... it will only be made available via new devices, not on existing devices." More info here.

Visual voicemail. Microsoft notes simply that "Visual voicemail isn't available for [sic? "from"?] all mobile operators." A different page reads, "Visual voicemail isn't available for [?] all mobile operators and it uses your cellular data plan. If it's offered by your mobile operator, it must be enabled on your mobile phone plan and your mobile operator might charge extra for it. Depending on your mobile operator and current phone plan, you may have the option to set up visual voicemail, which lets you see your voicemail messages in a list and play them on your phone." In my review, I wrote that most "people don't realize that Visual Voice Mail is not an Apple/iPhone feature; that feature was created by AT&T. So Windows Phone 7.5 does support Visual Voice Mail, but again it can only be offered when your wireless carrier explicitly provides it. So you can expect AT&T to offer this, and any other carriers that provide Visual Voice Mail. And unlike with the previous two features, it seems that this will come to existing devices pretty quickly."

Group messaging. Groups are a key new feature in Windows Phone 7.5. But Microsoft cautions that "your mobile operator or hardware manufacturer can choose how to configure this feature ... When group messaging is turned on, replies are sent as MMS (Multimedia Messaging Service) messages instead of SMS (Short Message Service) text messages ... If this setting doesn't appear, replies to the group will always be sent as MMS messages. Your mobile operator might charge extra for MMS messaging, and this service must be available with your current phone plan."

Camera focus. This one is simply amazing: "Your mobile operator or hardware manufacturer can choose not to support custom focus (the ability to focus on the point you tap) ... In this case, the camera will automatically focus on the center of the screen." I'm not clear on why a wireless carrier would be given the ability to disable custom focus, but I suppose it's possible that some camera hardware doesn't work properly with this feature. Anyone?

Clickable phone numbers. Another oddball optional feature: "In the United States, you can click a phone number in text to dial that number. This feature is not available in other countries/regions."

Finally, Microsoft notes that its own document "is not a comprehensive list of all feature and service experiences that differ by language, locale, mobile operator, or hardware manufacturer." Right. But that's why Andrew Birch's Windows Phone 7.5 feature tables exist. Check 'em out.