low level audio

for some reason the phone recoridng i am doing has long drop outs. playing with audacity there appears to be a signal at -48bd. but i am not sure if its background noice or a faint signal i can amplify. does any one know how to amplify or retreive a very low level signal at -48bd?

Select the bit you want to amplify (click and drag with the mouse)
Then use “Amplify” from the Effects menu.