Simplify the following text for a Grade 7 native English-speaking person.
Original: Prior to ISDN , the phone system was viewed as a way to transport voice , with some special services available for data . The key feature of ISDN is that it integrates speech and data on the same lines , adding features that were not available in the classic telephone system .
Simplified: ISDN offers a number of services , which use the telephone line to transport data .