noun
prepaid cellphone
A mobile phone that uses a prepaid plan rather than a monthly contract. The term is straightforward and commonly understood.
海外旅行のときはプリペイド式携帯電話を使っています。
When traveling abroad, I use a prepaid cellphone.
Compound of プリペイド (prepaid) + 式 (style/type) + 携帯電話 (mobile phone). A straightforward modern term.