In practical scenarios, a T.38 fax call has at least part of the call being carried over PSTN, although this is not required by the T.38 definition, and two T.38 devices can send faxes to each other.
In real-world scenarios, a T.38 fax call frequently relies on PSTN infrastructure, although this is not mandated by the T.38 standard. Two T.38 devices can exchange faxes with each other.