Context does matter here. If the caller would reasonably expect you to answer the phone (such as if they were calling a store or other place of business), an apology for not being able to get to the phone makes the most sense.
While you do the manual footwork, make sure to implement a system that automatically handles your client follow up via email to set yourself up for success.

The most professional voicemail message should include a formal tone and specific instructions. For example, you may say “Hello, you’ve reached [your name], [job title] at [business name]. I’m sorry to have missed your call. Please leave your name, contact information, and reason for calling so I can get back to you promptly.”
If you don’t know it by now, you might never know. Voicemail greetings are incredibly important to users. It conveys identity, character, and image to callers. In an increasingly digital world, where business is conducted online and over the phone, phone presence is increasingly important. As such, your greeting is a crucial part of the platform. While there are a number of obstacles to be aware of, utilized properly, voicemail greetings can be incredibly helpful and beneficial; therefore, its in users best interest to invest themselves in creating a quality recording.