If I’m using the phone for normal tele-conversation, will that decrease my bandwidth or ADSL Internet access speed when downloading, and by roughly how much?
As the voice communications over POTS (plain old telephone service) operates at frequencies below 4KHZ and the frequencies for the ADSL modem is higher than the one use for voice communication, the bandwidth available to ADSL will not be affected during your Internet access via ADSL modem, thus it will also not slow down or decrease your access speed.