So, there is a 3rd way - instead of losing customers to OTTs, or build your own (and sometimes fail, such as the case of Telefonica and Tu Me - here), MNOs can outsource such services (e.g. "Bouygues Telecom Deploys GENBAND's fring OTT Service" - here).
A recent report by Declan Lonergan [pictured], VP, Yankee Group finds that "The growing popularity of over-the-top (OTT) communications apps such as WhatsApp, LINE and Viber poses serious challenges for mobile network operators (MNOs). As operators consider how to respond, one option is to introduce their own OTT apps by partnering with vendors on a white-label basis. This approach should be considered seriously, particularly by MNOs and mobile virtual network operators (MVNOs) that lack the resources and/or time to develop an app in-house.
.. White-labeling is a viable option. To respond to this growing threat, MNOs have several choices. One option is to introduce an OTT app under their own brand by white-labeling a solution from a growing number of vendors. This is a valid approach that allows MNOs to deploy their own apps quickly and at relatively low upfront cost".
See "Can White-Labeling Solve Telcos' OTT Challenges?" - here.