DD is 2. Every time I talk to someone else she will start shouting ‘mummy, mummy’ getting louder and louder until I speak to her - and even then she doesn’t always have anything to ask, she just wants my attention so continues to repeat ‘mummy, mummy’.
The first time she says it while I’m talking, I look at her, smile and say ‘Just wait a minute, I am talking to XX’ - but it has no effect. I have had a talk to her about waiting her turn to talk and she now sometimes replaces ‘mummy mummy’ with ‘I’m waiting, look how good I am being waiting. Mummy! Are you proud of me?!’ 
Anyway, not the hugest issue but if anyone has successfully trained their child to wait patiently I would love to hear how!