The basic rule of thumb is that callers should hear one of two things when they first connect with your voicemail — either an apology for not being able to answer the call or a “thank you” for having called. You can do both if you prefer, but keep it short and to the point.
@Fa5t3r - I would try a wide range of different tones when recording and play them back to see what goes over the best and what projects the image you want people to hear. Remember that some people may be going to hear this message over and over again depending on how often they have to call you so you want it to be very pleasant and not at all irritating.

Because telephone calls are far more likely to reach you than scheduled appointments, the organization must have a system in place to intercept incoming calls to ensure they are turned into actual leads. A customized voicemail transcription for business calls directly affects the way customers perceive your brand. It sets the tone for every conversation that follows and is often the first contact with your brand for a potential customer. All of this builds on the power of the voice to build on your customers' relationships. The immediate resolution for any issues with transcripts of voicemails in hand lets small business owners keep on top of any client experience, maintain a pleasant presence, and remain coordinated.
It is courteous to acknowledge the effort the caller has made to try to connect with you over the phone.