What happens here is when you apologize and sound genuine in your tone, your prospects would understand and leave their contact details for you to follow with them.
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.

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. Ho-Ho…Hello!: 5 Ways To Spice Up Your Company's Holiday Voicemail Greeting 10 Sure-Fire Ways to Engage Callers with Your Business Voicemail Greeting Home Services Live Answering Service 8 Key Elements of a Proper Business Voicemail Greeting
8.) Benvenuti al Servizio Assistenza della John Doe. Purtroppo, tutte le linee sono occupate al momento. Si prega di lasciare un messaggio dopo il segnale con il vostro nome e numero di telefono. Vi richiameremo al più presto possibile. Vi auguriamo una buona giornata.