I think the love between your husband and that woman is special. If your husband doesn't like her or love her, he will change his mobile no. right away. There must be some kind of special feeling between them. I don't understand why your husband showed the SMS message to you.... let you accept the fact so that he keeps on receiving the message every year?
I believe that woman only wants to express her love to your husband once a year. I just cannot accept that your husband keeps his mobile no. and continues to receive her affectionate birthday message every year.
Before my hubby's ex-gf would call him once in awhile, sent SMS, asked him out and said regret words.......my hubby stopped receiving her calls and she used 133 to dial (private call, no number showed), my hubby stopped answering private numbers until WE DECIDED TO CUT THE NUMBER.
That way she can't disturb our life and my hubby showed his love to me becos he felt annoying and the most important thing is what she did annoy me the most.
I think you should talk with your hubby about your feelings. Upset is upset, no need to hide. Get her out of your life!!!