Vista 390 Extension in Use problem

I had this exact same problem. First to thing to check is that all other equipment is functioning; it is possible that the receiver on a fax machine has been knocked off its cradle.
Also the wires at your jack should be tested.

It was intermittent. It took a few years, but we finally connected the fact that this only happened after there had been considerable rainfall outside. (... I know!) The Bell repairman was able to determine from outside that the problem was not inside; he had me disconnect each and every piece of equipment and he checked the strength of the signal after each one was removed.

Then he told us the truth; our line was strung up as a 'temporary' line back in 1985!! Bell was replacing trunk lines on city streets to improve a decaying infrastructure and would have replaced all temporary and fraying wires as each street was upgraded. But - according to the repairman who has been with Bell for decades - Bell stopped the upgrade projects in favour of putting the funds into competing with Rogers and the internet. Their philosophy since then has been to make temporary repairs to provide a phone connection no matter how bad, and regardless of how many times they have to make service calls. There are literally tens of thousands of customers who are receiving bad service out there, because a) their main line has been 'temporarily' spliced into a neighbour's existing line, even tho' they all pay full service rates!, and .... b) thousands of street-long trunk lines have frays in them that short out in the rain, scrambling service for everyone along that line, to some degree, but there is currently no one working on system upgrades. There would have to be literally scores of complaints along a particular street for Bell to even think of upgrading the trunkline.

As soon as we had a permanent phone line installed, the 'extension in use' problem stopped.

So please have the outside wires checked out and ask the repairman questions about the condition of the actual lines.