How to let my husband's secret admirer know I know something is up...?

We used to own a rental property and one of the tenants apparently had/has a thing for my husband. We sold the property in May, but she calls occasionally (once every couple of months) and leaves messages on his phone to the effect of "hey, just wanted to say hi and see how things are going, blah blah"). He returns these calls, but his are short in duration. I've called him out on this several times and he denies anything is going on (on his end) and swears he's told her not to call again. Whatever. What I think about that is a story for another day. He could be having an affair with her at this point for all I care, but that is not my question. I am just trying to come up with some clever way for a male friend of mine (he's just my hairdresser) to call and leave a message on her voicemail that will make her go "hmmm, that's weird". Just something to raise an eyebrow. Something they can sit and stew over, but I can plead ignorance to. Any ideas?