diànhuà lǐ tīng bù qīng kěyǐ shuō shēngyīn xiǎo xìnhào chà huánjìng chǎo yǔsù kuài

电话里听不清，可以说声音小、信号差、环境吵、语速快。

If you can't hear clearly over the phone, you can say the volume is too low, the signal is weak, the environment is noisy, or the speaker is talking too fast.