To be an mobile operator was simple in the past.
You sold a postpaid or prepaid subscription with a subsidised mobile phone. You charged the subscriber for each voice call, sms and mms. Eventually could the customer add international roaming and/or a dataplan at a high cost.
Welcome to Sweden today. Family plans and installment plans for phone of you choise. A customer have several subscribsions, each of them with a separate SIM-card. They pay a fixed monthly fee and get free voice calls, free SMS and free SMS.
They have a monthly datacap for 4G for all subscribsions together, i.e. several SIM-cards. International roaming is getting more and more regulated within EU. No real possibility to block services on top of data traffic, makes competition tough.
The future is already here. In the release notes for WatchOS 2.0 there was the possibility to make voice-calls over wi-fi with some operators. Suddenly, the physical SIM-card is as obsolute as a CRT.
The question for the telcos is not in they can fix the networks, as they will. The real is question if their IT-systems are flexible enough to handle the changes to the business model.
Update from Apple support pages:
If you're in the United States and use T-Mobile, you can place a Wi-Fi call from your iPad or iPod touch with iOS 9, Apple Watch with watchOS 2, or Mac with OS X El Capitan. When you place the call, your iPhone could be right next to you or far away—it could even be off.