On one level, the answer seems self-evident: the phone is an I/O device, and should be treated as such in the architecture in general. It is like a keyboard, or a mouse; it is just an audio I/O device.
However, at a deeper level, the answer is not so obvious. To access an enterprise’s applications, there is an assumption that a keyboard and mouse exist – there is an assumption the user has a computer! Last I checked (most) computers don’t come with telephones.
It turns out this problem is harder to solve than it seems. The current crop of phones assumes that the telecommunications complex is an island unto itself. Softphones are PC applications that leverage a headset to turn the computer into a phone that connects to the PBX. PBX vendors sell “hard phones” and there are adapters to turn an analog phone into an IP phone that once again connects to the PBX. Cell phone and PSTN phones suffer from the same malady.
