Hi IlPadrino,
Now that you have the codec installed can GSpot also play the file when you click the number 2 button in "MS A/V" ?
I downloaded the G.711 codec from your link, but I don't have anything to test it with. (I can't find my old sample files and my current telephone company does not provide such a service.)
Could you post a small sample file? You can attach a zipped wave file to a reply. (Click the Additional Options... text to access the Attach dialog when replying.)
EDIT
I have actually archived some old memorable voicemail files. Back then I converted them to uncompressed wave files with dBpoweramp. The converted files are universally playable and not significantly bigger than the compressed a-law files were (a-Law is the European variation of u-law). This is what MC reports about them:
Uncompressed Audio File (PCM)
8.0 kHz, 8 bit, 1 ch