The use of function calling in large language models (LLMs) enables developers to connect LLMs with external tools and APIs, enhancing their utility at specific tasks. However, evaluating the performance of function calls in LLM pipelines is becoming increasingly critical as more applications are deployed into production. Evaluating function calls involves examining each step of the process, including routing, parameter extraction, and function generation. An open source library called Phoenix offers a built-in evaluator to measure the performance of function calling within major LLMs, providing a tool for tracing and evaluation.