Pentarrhinum insipidum belongs to the family Apocynaceae. When it's not in flower, it looks similar to the invasive Moth Catcher, a vine that comes from South America. Heart Vine is indigenous to Africa and it is found twining in among grasses but it can also grow aggressively in disturbed soil.
I took the picture above of the same vine in November, before the summer rains had started so it looks a little different. The fruit, stems and leaves are all edible and when it seeds, I am going to plant this in my vegetable garden, since the traditional vegetables are all badly affected by root knot nematodes, aka eelworm. I have noticed that many native plants are not affected and I hope that this is one of them.