'How about .." could also be expressed as :
Wie wäre es (mit )....?
Wie wäre es ,wenn wir ihn anrufen würden ?
How about calling him ?
Wie wäre es damit ?
How about it ?
As for "what about " in the context of the examples you gave and that Fab gave a great translation of , that is not the 'what about ' to compare with ' how about ' . 'what about 'could have almost the same meaning of 'how about ' with a slight difference in connotation . When you say 'how about ' you are offering a suggestion ,that you find favorable from your point of view , while ' what about ' is used to express the concern about a matter as an objection of the before mentioned statement .
"How about we go to the cinema ?" Wie wäre es mit .... ( here it is a suggestion )
- Sure I would like , but what about the unfinished work we have to do ? " Was ist mit ... ? ( here it is a concern contradicting with the before mentioned ) .