It has happened already and there's no going back. Nevertheless, your husband should know bout it no matter what. On the other hand, its not that advisable to tell him on the phone. It should be a face to face thing and I hope you will be well prepared bcos it might not end up well.
Hope this is your first and will be your last time of making such a mistake.
You are married and there should be nothing like a 1-time thing even if your husband has to be away for 1 year, you signed up for it when you said "I DO".