Network and infrastructure
Codec
A codec is the algorithm that digitises and compresses voice so it can travel over a packet network, then rebuilds it at the other end.
Every codec trades bandwidth against fidelity. The most frugal keep only what makes speech intelligible and discard the rest, which is enough for a conversation. At the start of a voice over IP call, both ends negotiate the codec they will share, among those each one accepts.
Why it matters for fax
A codec that compresses speech destroys a modem signal, whose information lies in the exact shape of the wave. Fax only survives an uncompressed codec such as G.711, and then only on a stable network, or a dedicated transport that simply avoids passing fax off as voice.