A800 limitation help needed

hi , i have a quintum a800 running on a adsl line 1mb/256k. everything seems fine when 1-2 people are calling , when there is up to 4 people calling , it seems like adsl router gets heavy load and crashes , resetting the adsl connection. is there a bandwidth problem or is it just the adsl router (ecitele b-focus 400pr router) . when 4 people are calling and i ping the adsl router from the pc the pings are around 800-900ms. am using g.729 codec. please help.