母

How it works

が (subject) marks 母(はは) (mother) as the one doing the action. 呼んでる(よんでる) (is calling) is the casual, contracted progressive — full form 呼んでいる(よんでいる) (is calling), with the い ((dropped)) squeezed out in speech. The final よ (you know) adds a heads-up tone, alerting the listener to something they should act on.