First; it's not a plum; it just carries that name. The flowers are sterile so will not set seed or fruit. Parts of the plant are also poisonous. Being Apocynaceae; it is more closely related to the Jasmines. A fine ornamental, but not an edible.

I have only seen 1 with fruit. But never tried to eat it.
I am a wimp!

Natal Plum is very edible... But fruiting can be a problem especially if there are no other Carissa around. Maybe you could try hand-pollination or, like I did, plant another bush nearby.
Here is a link to Natal Plum.
Rose-Marie

Here is a link that might be useful: Natal plum

Especially if you live near Rockport TX on the coast - eating the ones grown wild here would be particularly nasty!

Umm the reason it would be nasty is that my little girl Hattie (8) and I love to drive around, pick them and eat em! We would be really upset if you chose plums that were bright red and just a tad soft to the touch!

I just dug holes to plant 50 of them for her 9th birthday coming up next month. Anyone know what soil I should add- What we have is a bit sandy.

I have lived here since 1998 and enjoy the lovely strong frangrance that these plants with shiny round dark green leaves(new in 1997) give around my pool. One just gave a bright pink fruit which is smaller than any of my limes but I have no idea how or whether it is eaten. Does the one in TX have fragrant white star flowers?

It's not true.....this plant gives edible fruits and even enough tasty. Full in half-shadow and protect it from freezing temperatures :)