remain unpaid
The Spanish phrase 'quedarse sin saldo' is often translated into English as 'to run out of balance'. This expression can be used in various contexts such as finance or telecom where one can run out of monetary balance or data/voice call balance. Literal translation might give you 'to remain unpaid', but that doesn't carry the idiomatic essence as accurately.
I ran out of credit and can't call.
This phrase expresses that the speaker has no remaining balance on their phone, preventing them from making calls.
I didn't realize I had little credit left.
This sentence indicates that the speaker was unaware of the low balance on their phone, which is essential for making calls or sending messages.
If I keep spending so much, I'll run out of credit soon.
Here, the speaker is warning themselves that their current spending habits will lead to a depletion of their phone credit.