Traditional telephones use "carbon granule" microphones that did not change much over 100 years. They are inexpensive and reliable, but they have one problem.
In humid weather, they can absorb moisture -- like salt or kitty litter -- and the granules clump together. The volume of your voice is reduced and both you and the other person hear a tell-tale shooshing sound.
There's an easy FREE repair that anyone can do.
Just take the handset and whack it on a hard surface, like a desk or a table, and the impact should separate the granules so you can have a normal conversation until the next deluge.
It's a good idea to protect the whacking surface with a magazine so you don't damage the phone or the furniture.
Photo from iStockPhoto. Thanks.