CODECs are located at the boundaries of the digital and analogue networks.

Fortunately, you can make calls between the two networks. For this to work

The CODECs inside the network and the telephone must use the same rules when formatting the bit stream that represents the users' voices. Provided both devices doing the same processing, then the information can be converted by applying the same rules in reverse.

Given the presence of the CODEC in the network, and adherence to the correct protocols, any device that can be used on the PSTN, such as a modem or a fax machine, can also pass calls into the ISDN.