Kia, méfie-toi du mot 'ignore' en anglais - c'est un des 'faux amis' entre les deux langues. En anglais 'ignore' signifie 'ne prêter aucune attention à quelqu'un' ou 'feindre de ne pas voir quelqu'un'. A simpler way of saying 'not aware' is not to know, or not to realise something. You could say perhaps : 'Sorry, but I didn't know who the message was from/I didn't realise who the message was from.' Or you could also say 'I had no idea who..'

Ignore means the same as ignorer in French. You want "to know". In your example you need to say "Sorry I DON'T KNOW who this is. I changed my phone and lost my contacts." As a more general example, if you don't know something has happened, you can simply say "I did not know".

you can say: Sorry I didn't recognize you.