It was a 'make a decision ... fast!' situations.
I did think about telling him it was 'me' he was talking to, then I thought me might hang up, and be embarassed to call again in case I answered. plus it would make our next meeting really awkward so ...
I decided I wouldn't identify myself unless he challenged me - I wouldn't dream of deceiving him. He never did realise it was me - I suppose you never expect to encounter someone you know - and to this day he doesn't know we discussed his marriage, and that's how it will stay.