The lines, run by NHS mental health trusts, receive over 200,000 calls each month.
But according to research conducted by the broadcaster following Freedom of Information request, at least 418,000 calls went unanswered in 2021-2022.
Only 29 of the 47 mental health trusts responded to the data request, meaning at least one in five calls at those trusts did not receive a response.
In 10 trusts, some calls waited more than an hour for a response.
One caller, named by the BBC as Hannah, said she was initially told to think “happy thoughts” and to read a book when she called in a crisis after repeatedly trying to get through.
"I’ve literally been crying my eyes out and I’ve left a message on the answer phone and no-one’s ever got back to me,” she said.
"Your brain already makes you feel that you’re alone. And then to have the people that are meant to care not answer, it makes it 10 times worse."
When she got through, a staff member allegedly blamed her for “not wanting to help yourself” and ended the call.
South London and Maudsley NHS Foundation Trust has apologised, saying improvements have since been made and that advice given to Hannah was not appropriate.
Former staff members handling the calls at different NHS Trusts also told the BBC they were not given training on supporting patients, and felt limited in the time they could talk to patients to meet demand.
In 2022, coroners investigating published two Prevention of Future Deaths reports - written to highlight risks that could lead to future deaths - that highlighted concerns about crisis lines.
Prof Sonia Johnson, an academic from University College London, told the broadcaster she believed staffing pressures in the wider mental health system have led to demand for crisis services among patients unable to get the long-term support they need.
A Department of Health and Social Care spokesperson said by 2024 its workforce investment would “deliver an additional 27,000 mental health professionals and give two million more people the help they need".
NHS England said crisis lines had seen “record demand” and that it had made £7million available to local areas to improve their crisis lines.