Ma'am,
Your mother's husband is in charge.....and I don't believe telling her something, even if it is the truth at this time is acceptable for a smooth transition. Let her husband talk to her, and if he feels she needs to hear she is on hospice, he will do what is right. If you call an ambulance, the ambulance driver will ask if she is on hospice, and will refer your mother's situation to hospice, anyways! Let this passing be a peaceful, supportive path! Read to her, sing a hymn, hold her hand or even sit with her and watch a movie, that she likes. Try something that can deter her mind, for you know the prognosis already!