How should I reply the company?

One of the company (Company A) that rejected me after bringing me on-site interview just emailed me and asked if I'm still looking for job (no official offer letter yet) as they have a vacancy for the position I interviewed for. I believed the candidate they gave the offer turn down the offer.

However, I have another phone interview tomorrow (first round) for another company (Company B) which I have no idea if I'll be able to move on to the next stage.

Company A is a small company while Company B is a big company. So the pay for company B will be higher than A.

What should I tell the company A?

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