Carriers have been looking for key areas where over-the-top firms will be forced to work with them, retaining their power in the mobile value chain. They have focused on technologies that are tied to the SIM card (M2M, mobile payments) or the network (customer behavior tracking), but perhaps the most powerful is the simple monthly bill. Carrier billing is proven to drive additional purchases and service uptake because of its convenience and has been incorporated in many app stores. Now it is being harnessed by web providers to improve their appeal to consumers. This week saw Skype extending its carrier billing to Russia through technology from Mach and Payvia and the feature will soon be available to Skype users in…