Have you looked at the history on FB and had also had a look at his search engine history generally? A name could crop up there. Maybe he's just found an old female friend that he's not seen for ages. Maybe she's had fertility treatment too and he just needs someone to talk to. I'm just wondering if he feels guilty about this new friendship. I would check out the app truecaller though.
Has he changed any other habits like the way he dresses, whether he's more attentive to his body image and smell and using and using loads of deodorant and aftershave. I'd go through his pockets too, regularly to check for odd receipts. He's bound to trip up sometime. Is there any unexplained expenditure on his Visa card statement or bank account statement? .
As OP suggested in an earlier post I'd definitely use protection when you're having sex; you don't want to catch anything. As for his comment re chlamydia, I'd pop to the GUM clinic and pick up a leaflet. Leave it around and if he asks, say you picked it up at the doctors after your conversation the other day. You wanted to read more about it. I'd watch his reaction very carefully.
To ask a really pertinent question, but an important one in my view, is your sex life. Has it changed recently? Is it what's normal for you as a couple or has he changed there? Putting it off?
As for phoning him I wouldn't. Trying to carry off an accent isn't easy under pressure and a women's voice... I'd get an old male friend to ring it for me. Get him to phone for a 'Ms. Ummmm, can't quite make out her signature?' If they say no, then ask could it be confused with the person at the end of the line. They may say no, it's "Miss/Mrs Bloggs".
What does he do at the weekend? Does he spend any time out, alone? Any change of habits there?
My hubs picked up a number, I rang, and came storming up the stairs not too long ago, because I made a call with a charge rate. It was actually the Samaritans, I told him, looking at him straight in the eye, as it was true. My answer obviously threw him, but I pointed out to him that having been recently sexually assaulted in hospital, I really needed someone to talk to, outside of my immediate family. I was getting sod all support from anywhere else and it WAS very traumatic. It was a helpline. Maybe he's depressed too? It's quite plausible as many men can't face it to talking about depression. Has he had any odd, random doctor's appointments, recently?
Cou,d he be planning something for you for maybe a significant birthday or wedding anniversary? I'm just trying to think outside the box!🤔
Hubs and I have joked that we'd chop it off if we found the other having an affair, but that's actually impossible for me to have anything chopped off. I'd be tempted to say a friend of yours said that she'd do the above, which isn't quite true, but true enough for this. And then watch his face. I joke about doing a 'Bobbit job on hubs, if he strayed'!😵 In reality though, he knows that I'd take him to the cleaners and get every penny I could.
I hope this is resolved for you soon; it's the not knowing that's worst. Going to absolute extremes I'd even go as far as to employ a private detective. Better than catching a serious and untreatable STD, definitely. Thinking of you.