Microsoft is to ban apps that enable VoIP services over carriers’ networks from its Windows MarketPlace for Mobile store when it launches in the second half of this year.

The VoIP app restriction is among the 12 application types forbidden in a list of "rules" announced by Microsoft.

Earlier this week, a Gartner report forecast that traditional network-based mobile carriers face the real prospect of losing a major slice of their voice traffic and revenue to new non-infrastructure players that use VoIP.

While the Microsoft ban is limited to VoIP services it is unclear whether developers will be able to offer VoIP apps using Wi-Fi.

That will not be known for sure until the store is up and running with apps.

InformationWeek points out, however, that Skype already works over 3G on Windows Mobile phones.

It suggests the ban on them must be a concession that the carriers wrung out of Microsoft in return for carrier billing.

Other prohibited apps include those larger than 10MB, those that promote or send users to alternative app stores for the same application, and ones that change the default browser, search client or media player on the device.

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