Update for: https://ngengesenior.medium.com/how-to-usevisualtransformation-to-create-phone-number-textfield-and-others-in-jetpack-compose-f7a62f8fbe95

OffsetMapping.kt

val cameroonNumberTranslator = object : OffsetMapping {
    override fun originalToTransformed(offset: Int): Int {
        // [offset [0 - 2] remain the same]
        if (offset <= 2) return offset
        // [3 - 5] transformed to [4 - 6] respectively
        if (offset <= 5) return offset + 1
        // [6 - 8] transformed to [8 - 10] respectively
        if (offset <= 8 ) return offset + 2
        return 11 // Total number of digits, plus two hyphens
    }

    override fun transformedToOriginal(offset: Int): Int {   
        if (offset <= 2) return offset
        if (offset <= 6) return offset -1
        if (offset <= 10) return offset - 2
        return 9 // Total number of digits, without two hyphens
    }
}

PhoneNumberVisualTransformationTest.kt

class PhoneNumberVisualTransformationTest {
    private val transformation = PhoneNumberVisualTransformation()

    @Test
    fun transformJustZipCode() {
        val input = "111"
        val output = transformation.filter(AnnotatedString(input))

        assertThat(output.text.text).isEqualTo("111-")
    }

    @Test
    fun transformZipCodeAndFirstThree() {
        val input = "111222"
        val output = transformation.filter(AnnotatedString(input))

        assertThat(output.text.text).isEqualTo("111-222-")
    }

    @Test
    fun transformStandardPhoneNumber() {
        val transformation = PhoneNumberVisualTransformation()

        val text = "1112223333"

        val output = transformation.filter(AnnotatedString(text))

        assertThat(output.text.text).isEqualTo("111-222-3333")
    }
}

Great. The unit tests are comprehensive and simplified