We were called a while ago by a lady who sounded like she had a nice voice and wanted representation by BigFish Media as a voiceover.
Us: "So do you have a showreel?"
You really must have a professionally made showreel before you approach a voiceover agency.
It is like asking to be a model without a portfolio of photographs.
We also recommend that you have at least two years experience of getting yourself paid voiceover work.