I've stressed it throughout this article, and I'll stress it again — your sales voicemails have to be concise and airtight. That requires being locked in on exactly what you're trying to get across. You can't go off on a series of tangents that may or may not apply to your prospect's situation. Keep your messaging as focused as possible.
This point is more or less an extension of the point above. If you want to avoid sounding awkward and long-winded, you need to know what you're going to say — every good sales voicemail is conducted with confidence and definitive intention. The key to delivering on both is having well-prepared, personalized voicemail messaging at your disposal.

“Keep your voicemail messages short, sweet, and snappy," advises Will Craig, Managing Director of LeaseFetcher. "Don’t try and go into the nitty gritty details of the discussion you’d like to have with them—save that for your actual chat. Alternatively, don’t leave them in the dark about what you want to talk about either. A simple 20-second voicemail letting them know the nature of your query will suffice."
Your own answering greeting should be short. Don't bother to say that you aren't available to take the call. That's a waste of time, and there's no point in restating the obvious. Instead, simply identify yourself and ask the caller to leave a message.