Complex sentence: Access' digital phone service is interesting in that it is provided by an ATA (Analog Telephony Adapter) with a built in cable modem that communicates over cable lines to a private IP (Internet Protocol) network.
Split into simpler sentences: Access' digital phone service operates on a device called an ATA (Analog Telephony Adapter) with a built in cable modem. The modem communicates over cable lines to a private IP (Internet Protocol) network.