in last few weeks after monsoon started i have ran my 500combo connection with downstream snr of ~7 with no dial tone in phone at full 2mbps.before monsoon my snr used to be at least ~23-25.but i know it is a line problem because upstream snr is still usual at ~20.still i can confirm that even with this low snr you can perfectly get expected full speeds on a 2mbps connection.what matters is the fluctuation in snr value(open modem info page & keep refreshing it continuously).if snr value changes within a range of 5-6(say 7-13) while never less than 6.0 then even if phone doesn't work net will work just fine.
Thanks for the info but with such low SNR margin I've faced frequent link unstable issues .