A: My mobile phone is out of credit.

B: Here, use mine.  With calls billed on a post paid basis, (固定收費) you never have to worry about running low on value or have to top up your calling card.

A: Yeah, you bet.  Do you know where I can top up my phone?

B: You can buy a top-up card in any convenient stores.  Or you can top up your mobile phone on the internet – just go to the website and buy credit for your phone with your credit card.

除了top up，還可以用recharge。
　　
Your calling credit is low.  You may need to recharge your account.