Technically yes, the API supports PCM recording with sufficient quality to express an 18 kHz sine wave. Standard POTS lines and even T1 lines usually work with 8 bits 16 bits, which is mathematically sufficient to correctly record a maximum of 4 kHz. And since the microphone is usually not used for anything of a higher quality than the sound of telephony, I bet that hardware will be the limiting factor ... your mileage will definitely vary from phone to phone.
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