Just getting this into My Postings to find it, as I'm having similar problems sporadically lately. My error messages vary - Unable to establish connection/no answer/line busy. Then mysteriously OK. Maybe the problem's somewhere else, and I don't need to correct anything (except impatience)?
I'm on pay-as-you-go dialup with Eircom (Ireland's equivalent of BT).
What sometimes seems to work with me is changing some ISP setting, changing it back again & resetting. But maybe it's just the lapse of time that does it!