Grammar fix
top up
add credit/money to a phone or card
to add more credit or money to a phone, travel card or pay-as-you-go account
phrasal verbB1core
Hear it in context
Notice where the phrase sounds natural.
I need to top up my phone before I run out of data.
Can I top up my Oyster card here?
Fix the pattern
One small grammar move makes it sound right.
Don't say 'fill up my phone' for credit — it's 'top up'.