This also means "They should call", the subjunctive of kuita with wa- as the subject prefix.
There's a question on another one of the questions in this lesson that touches on what you've said. Is this true for all the words that are prefix-verb root-e? Can they all be either "verb prefix-as-object" or "prefix-as-subject should verb"?
It depends if the verb NEEDS an object. If it does, it should be clear. Also, some prefixes differ for subjects and objects.
aangalie = s/he should look
mwangalie = look at her/him / you (all) should look
In spoken language, is there a tonal difference or is it solely dependent on context?