Block from U+A700 to U+A71F. This block was introduced in Unicode version 4.1 (2005). It contains 32 codepoints.
The Wikipedia provides the following information on block Modifier Tone Letters:
Modifier Tone Letters is a Unicode block containing tone markings for Chinese, Chinantec, Africanist, and other phonetic transcriptions. It does not contain the standard IPA tone marks, which are found in Spacing Modifier Letters.