panicandanxiety - when it comes to mobile phones, they're so easy to pick up on Ebay for a low cost, with or without a SIM.
I have (over a number of years, I'm not an anorak mobile phone collector!) bought a few Nokia 8310s (small with a sensitive FM radio).
Several have come with SIMs in. However, in many cases, it is easy to get SIMs for free with little or no trace (again via Ebay).
I really wouldn't waste any time on trying to get a trace on the number to identify anyone - it could be unregistered (like the 2 Asda SIMs I have), or registered to a business (without knowing who within the firm is allocated that number). Even if it had been registered, it could be at an address that was last used 10+ years ago (I have 2 Orange SIMs, one dating from 1996, the other from 2001), so you could spend money and have no information apart from a surname and initial and years of house moves to follow.
Also, having seen catwalkers experience, there's just no knowing how any other partner (assuming the OW is married) will react. If the OW's partner decides he will damage your home as a way of 'getting at' your husband, it will be you too that suffers, and no knowing how long it might go on.
Can see why 'being put in the picture' might seem a desirable thing (and anyone who had so-called friends who knew, but left them in the dark, will accept that thinking), but the unknown consequences are just that - unknown.
Sorry things have gone this way - hope you can settle things as amicably as possible.