OK, think logically about it.
If it was an escort as some have suggested, I think they would very very likely have an actual voicemail message saved instead of the generic one. They use their phone for their business as the sole way most wouold get in touch with them. So highly likely there would be a recorded message along the lines of leave me the best way to get back in touch with you or some such.
If it was scam, these 99% of the time contain a link to click on. They 100% of the time include some sort of urgency to the text. This is so people react quickly to what they want them to do. If simply a reply is what a scammer is after, it would certainly contain some sort of urgent need for the recipient to think they should reply. This contains neither. Its highly unlikely to be a scam.
Was there any xx's after the message? If an affair, it is highly likely the message would contain some sort of x or emoji or something like that. It was worded very straight forward.
My thinking is its either a wrong number, someone he has worked with on a project who just calls everyone babe (I know a few women who do this, its weird, but apparantly not uncommon), and is in the area and letting him know.
The only thing that stands out is the number isn't saved on his phone as anything. If it was an affair, wouldn't it be more likely he'd of saved the number as 'Dave the Plumber' or something? So I'd lean more towards a wrong number at this point.
I think I'd just be in touch with him today a little more than usual and if he seems to go out when he said he wasn't going to, or doesn't answer or get back to you in a reasonable time, then maybe just keep an eye out. It isn't necessarily anything bad at all.