SNRM glitches on phone use FTTC

Whenever I make or receive a phone call the upstream SNR Margin plot shows a momentary drop of a few dB at the beginning and end of the call when I pick up and put down the receiver.
Same results with several phones, different extensions, only 1 phone in at a time...
Extra filter make no difference.
Chart below shows SNR Margin on DSLstats with 10 sec sampling period.
Is this something I should try to investigate further or is it external and "not unusual".
( Step increase in SNRM on chart on upload is just reversing the step drop in SNRM I get everyday about 10:00 !)

[IMG]http://i59.tinypic.com/6oz91d.jpg[/IMG]

Small glitches can sometimes be seen on downstream SNRM

Modem/router is Billion 8800NL

I think the first question should be ... what problem is this actually causing you ?

None of course , just curious !
It may have always happened on this line - but have recently started using DSLstats to monitor the steady drop in SNRM since FTTC enabled a few months ago ( 14 dB drop up and down) and noticed these glitches.