In an ELL’s first language, if the sounds /b/ and /v/ are interchangeable, this is an example of:

The situation described, where the sounds /b/ and /v/ are interchangeable in an ELL's first language, is an example of phonological interference. This occurs when the phonological rules or sound systems of a speaker's first language influence their production and perception of sounds in a second language. In this case, because the two sounds are treated as equivalent, it may lead to confusion or errors when the learner is speaking English, which distinguishes between /b/ and /v/.

Phonological interference can manifest in several ways, such as substituting one sound for another during speech, which is exactly what's happening here. This type of interference is common among language learners, especially when their native language does not differentiate between certain phonemes that are important in the target language.