Yup, you only need one. I have only had 1 since I was 14 months old, when I lost one to cancer as a baby. Most people live quite happily on one - I have for 50 years - but unfortunately my other one had been deteriorating for the last few years. But your point is valid, if you donated 1, you...
TBH, I've not really thought about it (maybe I should), but I would hope they'd try to call me more than once. They have my home number, mobile and work land-line, plus next of kin etc., I can't imagine they'd try once and give up - but it's a good question.
I never used to answer calls from unknown number, but now I have to = I'm on the kidney transplant waiting list, so could get a call at any time, from any number. As it'll be Guys hospital, it will be through a switchboard and probably "unknown".
That said, I don't always hear my phone, which...