Well its just that if your broadband drops out at the same time you've probably got an intermittent connection. You're noticing this more with your voip because you lose the call. With the broadband you probably just load the page again.
Check for line noise on your telephone line, replace the filters, remove other devices from your landline (fax machines, sky boxes etc) and see if it still happens.
Any views expressed by me are my opinion and not necessarily the opinion of BT