"phone mam" i do not have a typo in my answer. i don't call my mother "mum".
You can now call her Mam in both languages - just added it to the English answer.
This sounds like one sibling telling the other to friggin' call mom, NOW! Which is .. a very .. hmm .. distant and formal sibling relationship there :P I would never tell my brothers in German 'Rufen Sie Mama an!' (formal 'you'), and I sincerely hope that Welsh siblings don't usually end up estranged enough to use the formal form of ordering each other about ;)
The -wch form is used when addressing an individual on a formal basis, but also any two or more people, familiar or formal.
So here, it could be sibling 1 shouting at siblings 2-5 because sibling 6 has just fallen over/been arrested/fallen ill with plague/etc.
I was mostly just kidding too but yeah LOL That could definitely be true as well .. I'll remember to yell that at my brothers next time I catch them together and confuse the ever-living hell out of them ;)