There appears to be a lot of confusion about the phasing out of ISDN2 phone lines, in my opinion mostly caused by alarmist Sales ploys. This mature technology is indeed being retired by British Telecom, who have set a withdrawal start date of 2020. At this point BT will no longer take new orders for ISDN2 and Customers must instead choose a technology such as VoIP. The ISDN2 product will be completely withdrawn by 2025 if they meet the schedule.
BT are doing this as the high operating cost vs low profit margins of supporting this reliable technology in a world where competitive VoIP has over-ridden the market. Installation and support Engineers must be properly trained for the ISDN2 network making it a relatively expensive product for BT. There are many businesses for whom the reliability of ISDN2 is not only essential but life-saving (NHS, Care Homes, Emergency Services, etc).