Hospice question, what specifically does it mean to "call in" hospice?

I agree with the previous post that you have to understand that hospice does not provide 24/7 care. Although often there are hospice facilities that will provide that during the last 1-2 weeks of life, when it is clear that the end is very near.

My MILs plan was to stay in her house till she died, using hospice at the end to allow her to stay there. This was not a realistic plan, because in the end she needed someone there 24/7 to help with everything as she became completely bedriddren and needed medication every few hours. She was lucky that my husband and I were able to fly out and stay with her. for the last few weeks, along with my SILs. And it was REALLY hard to do, even though we did have aides (paid for by medicaid) who were able to come in for a few hours every day to help.

If she had been in assisted living, or other long term care, most of them will work with hospice to provide end of life care. So if there isn't anybody at whom who can provide the 24/7 care you might look at other options.

My father died peacefully before his care needs became too great so it wasn't nearly as difficult taking care of him.

For my MIL and dad, hospice provided the hospital bed, the medications delivered to the home(pain and otherwise) along with guidance as to how to use them, bedside commode, social worker for advice, nurse visits 1-2 times weekly, massage (which helped my father a great deal), and if we wished counseling afterwards. Also a phone number to call with questions 24/7. We didn't have to drag MIL or dad out to doctor's appointments (that would have been so miserable for them).

Good luck figure out what will work for you. One thought is to contact hospice and talk to them directly. They will be able to explain what they can do. And every hospice organization is different, so make sure you understand what the organization you choose can offer.

Please note that hospice ASSISTANCE doesn't mean that they provide 24/7 care at the home. They provide telephone support and can arrange to either send someone out or provide info over the phone. If a family arranges hospice at home, the family and friends are expected to provide the majority of care.

In addition to geewiz's accurate description of hospice I'd like to add that once hospice is involved they will provide a hospital bed for your loved one. A hospital bed makes caring for someone much easier as it can be raised and lowered as needed and the back can be up for feeding or down for resting. Caring for someone who is in bed is much easier on the back when the person is in a hospital bed.

Hospice also makes available medication for pain and/or anxiety. This can be administered as needed by family or a healthcare worker.

As geewiz mentioned hospice also provides a bath aide who comes to the home a couple of times a week to bathe your loved one. The linens on the bed are usually changed at this time too.

Some families opt to have a nursing service in addition to hospice. The nursing service provides more comprehensive care as a supplement to the hospice service. The healthcare worker will be someone experienced in hospice and end-of-life issues.

Hospice assistance is available 24/7. If you don't have another healthcare worker in the home and you have a question or concern you can call your hospice provider at any time.

Hospice is all about keeping your loved one comfortable. If she wants to eat, she eats. If she doesn't want to eat, she's not forced to eat. Same with fluids. Hospice provides many supplies, among them sponge sticks which can be dipped in water and used to keep a person's mouth and lips moist.

There are no trips to the ER or the Dr. however if your loved one becomes sick with an infection a doctor affiliated with the hospice provider will prescribe an antibiotic. If she gets a UTI, again, an antibiotic will be prescribed. These medications provide comfort and will not prolong your loved one's life.

Experienced hospice workers can at times give you an idea of how much time your loved one has left if she's nearing the end. There are usually subtle signs and symptoms.

Once your loved one has passed away hospice takes over and makes the necessary phone calls. They come to your house and sit with you while waiting for the mortuary. They'll dispose of any leftover medication and assign the legal time of death. While this is not a happy time many people have agreed that hospice provides a comforting conclusion to what had been a sorrowful and stressful time for the family.

Medicare dot gov has a nice summary of hospice , check it out.
Briefly, once a family/patient realizes that there is no longer any treatment that will help or that the patient wishes to take, and that the situation is terminal - hospice can be requested (if the doctor agrees).
You select a hospice organization in your area and they provide an assessment. (note that each hospice organization offers different services so it is worthwhile to call the ones serving in your area to determine what each offers. You can do this in advance so you have your selection made in advance of needing it).
Once a person is on hospice, treatment for the cause of the impending death is no longer offered. The person is however made comfortable. Hence, a cancer patient for example would no longer take chemo or radiation but would get wound care.
Typically (in my area at least) an aide is provided to assist with feeding, dressing or bathing. A nurse comes on a regular basis -- could be every 2 weeks in the beginning, more frequently as needed. Spiritual care is often included for the patient and the family (this often continues for a year after the death.) 
Was there something more specific you were hoping for?