Do not get to atteched to translation of prepositions. Especially when English ask for and ask about are different Polish verbs. Ask for is prosić (o+ accusative), Ask about is pytać (o + accusative)
I don't understand why "I'm asking mum for a new mobile phone" isn't accepted. In both cases - the English and the Polish - you don't need to say MY mum as it's implied that she's your mum.
The accepted answers are: [cellphone/cell phone/mobile phone/cell/cellular phone/cellular/phone/mobile/mobile telephone].