Thanks endless. Your DH sounds remarkably like mine! He has said that if I had found out at the texting stage, it would have all stopped. The strange thing was that about the time all this started, I had a really unsettled feeling about life and couldn't pin it on one thing in particular. In the end, I decided it was work that was the problem and even considered a change of profession. I just knew there was something wrong and that I felt restless and unhappy. I would never in a million years have dreamt that it was because my DH had started an emotional affair.
In my case, the texting, E mailing and 'phoning went on for more or less a year before he met her and in retrospect, I kick myself for not tumbling this earlier, but after 24 years, it was the last thing I suspected.
You may have caught him in the nick of time, which is good news. On the other hand, I think it's strange that she put the 'phone down on you. Beyond genuine shock and horror at being confronted by an angry wife, I do think if nothing had gone on at all, she would be trying to get some message to you that nothing physical has happened. What is your real, gut instinct telling you?
The reason he wasn't telling you about this is horribly simple - he probably wanted an affair with her, emotional or otherwise. Given that you have always accepted him to be a gregarious soul who is comfortable with women, you have hardly gone into orbit if he mentioned that a young colleague needed a shoulder to cry on, would you?
How are they behaving towards one another at work then? What has he said to her about your reaction? Also, have you actually looked for evidence of another phone or SIM card? I know this is hard, endless, but they could both be covering their tracks a bit now with changed numbers, phones and SIM cards. Just make sure you've got the full picture, is all I'm saying.
Keep posting and be kind to yourself through this horrible time.