"why the entire circuit need to be certified and what damage can the circuit behind the chip, if possible, can do to the telephone line."

Using a pre-certified device like this is certainly beneficial, but what it gets you is a few approval check-boxes ticked out of many approval items. The datasheet introduction says:

"The MH88422 Data Access Arrangement (D.A.A.) provides a complete interface between data transmission equipment and a telephone line."

The data transmission equipment itself (i.e., your circuit, your application-specific device) must be approved too. For the sake of discussion, let's say your device is a simple auto-dialer. Some countries may have requirements that your equipment behave a certain way upon detecting certain network tones to progress through the dialing sequence. They may have limitations as to how frequently you can dial the same number over and over in the case of no connection if there is no human involved in the initiation and termination of auto-dialing (i.e., a device that decides to dial of its own initiative and is out of the control of a human supervisor). They may have allowances for particular types of auto-dialers (e.g. and alarm system dialer). This is all stuff that are application-specific elements of the approval process that have nothing whatsoever to do with your pre-approved DAA.

The list of issues goes on and on. What if your device has modem functionality? There are issues of V.25 answer tone, possibly guard tones, then the data transmit and receive tones for the various signalling speeds. You probably wouldn't get approved if you dropped in a Bell-103 modem (300 baud in the USA) when your country specifies V.21 (CCITT's 300 baud).

Now, I could be entirely wrong and things may have change drastically since I last designed a PSTN-connected device and an approved DAA may be all you need, but I sort of doubt it.