There are many ways for MNOs to reduce the threat from OTT services (mainly in VoIP and messaging) and/or try to monetize from delivering it. One popular way is to charge for its use, other is to block it.
The leading MNOs in Poland came up with a third way.
James Middleton reports to telecoms.com that "Polish carriers Orange, T-Mobile and Polkomtel have banded together with local developers and academic institutions to create an ecosystem designed to address the growing competition posed by Over The Top (OTT) providers. The operators have deployed an open service layer within their core networks to accelerate deployment of innovative and low cost communication services with a healthy balance between cooperation and competition .. Telecoms software firm OpenCloud delivered the open standards-based service layer to the carriers and claims the model has given those involved a sharper competitive edge to attract developers".
".. Using OpenCloud’s Rhino platform, mobile operator Polkomtel / Plus has been able to take advantage of an extended community of developers to create and implement more than 25 services. These include Office Zone, Home Zone, VPN, Freephone, Split Charge, Premium Rate, Who Called/Notify ME, Voicemail in Roaming optimization....from multiple developers including IMPAQ and ATTR"
Jeff Gordon, CEO of OpenCloud, said "To counter the threat from OTT players, and boost their revenues, operators need innovation within the network and not just in device apps. However, delivering innovation in the core network has, historically, been an expensive and slow process” (case study available here).
See "Polish operators open core to combat OTT threat" - here.