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.
If callers are getting a message saying that your voicemail is full but you have already deleted all of your voicemails on the iPhone and you can see in Settings that voicemails are not taking up any memory- dial *86 from your phone and then press & hold the 1 button.

Don' let background noise do the talking for you. Ambient noise on your recording could be sending the wrong message to your clients. The sound of your kids arguing in the distance could be the sign of a distracting work environment. Clinking glasses and restaurant chatter may leave the impression that you don't take your work seriously. When you record your voicemail greeting choose a quiet spot free from interruption, so your message is the only thing callers hear. It's also good to avoid large, cavernous spaces where sound verberates.
Calling just to "check in" isn't a relevant reason unless your prospect has specifically asked you to. It's just a lazy excuse to get someone on the phone and it hardly ever works. Instead, find common ground between you and your leads.