Sadly the technology often required a line to be converted from analogue to digital and two channels were needed for maximum speed, incurring additional costs. ISDN's advantages were that it ran through a stable and highly reliable digital network, meaning that you always got the performance expected; though ISP congestion could still hamper quality.
BT's consumer grade ISDN products (HomeHighway, Midband) could also connect to dialup ISP's.
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