When you ‘get on to’ someone, you make contact with them, usually by phone. However, to ‘get onto someone’ can also mean to confront them or ‘call them out’ about their behaviour or attitude, or something specific they have done to aggravate or hurt someone. You can also ‘get onto’ a task or address an important issue, usually when someone wants it done urgently or it is important. Another meaning is to shift attention to something: “I’ve no idea how we got onto this subject. That’s not what we were talking about.” You can also ‘get onto’ a team, for example, or anything in which you become a member or contributor.