Translation:They called each other.
Could one say 'They rang each other.' or is that too colloquial?
That definitely means the same thing.
Seriously, this zij/ze is killing me! Sometimes zij is accepted, sometimes ze (for plural i mean), can you please, please add this as a substitution?
Zij already is accepted, unless it is a type what you hear exercise. In that case only ze is accepted here due to the difference in pronunciation.