Don't assume

Allan, we had the same thing happen with British Airways. We got an email saying two legs of our round trip Tulsa-Rome flight had been cancelled. They suggested an alternative for one leg (Tulsa to Chicago), but not the other: Rome - Dallas! And left all the other bits of the journey in place.

Since we weren't sure that we'd be able to get to Europe within a calendar year, we needed a refund instead of a voucher, so yes, we had to call. It took several tries. I've had best luck calling early in the morning.

I guess in normal times, this is the way it works, notified that some segment of an itinerary has changed, you can accept it, or look at other itineraries. I would find it odd, even in these times, that an airline would independently cancel and then credit your account, they would want you to either initiate a complete cancellation, or a change that will work. I know, I know, unusual times, the UK won't let you in, everything closed,...but some flights are still running, there are still some people moving between countries for one reason or another, they don't know if you are one of those.

I've had best luck calling early in the morning.

Was that a US number or a UK number (i.e, better success at calling a UK number during US morning hours)?

We're fully expecting our flight to Mallorca early August to be cancelled so it'd be nice to know what times people are finding best to call, although I suspect that there's very little difference. In all likelihood we'll probably settle for a voucher rather than a refund as we visit every year so if that can be confirmed via e-mail then it will save some hassle.

Allan, that sounds positively dreadful. If any part of a round-trip itinerary is canceled, to what does Canadian law entitle you? Is WestJet playing by the rules?

Yes, call, let WestJet know that you consider the alternative unacceptable and see if they will give you a full refund. In preparation you should review Canadian rules:
https://otc-cta.gc.ca/eng/flight-delays-and-cancellations-a-guide

Refund

If an airline (large or small) offers alternate travel arrangements
but these do not meet a passenger's needs, the passenger is entitled
to a refund. If the passenger's trip no longer serves its purpose
because of the disruption, and the passenger is no longer at their
point of origin (for example, if they are at a connecting point in
their multi-leg ticket), then the airline must book the passenger on a
flight back to their point of origin and refund the entire ticket (as
if no part of the trip had been made).

In any other case, when the alternate travel arrangements do not meet
a passenger's travel needs, the operating airline must refund the
passenger for the unused portion of the ticket.

Airlines must make refunds to the person who purchased the ticket in
the same form of payment the ticket or additional service was
purchased.

Here's a video about refunds if studying Canadian rules are too stressful.
https://www.youtube.com/watch?v=_IExD-J4_F8
Breaking Away: "You got it on paper?.... Refund, refund, refund"

JC, the two times I had to call I called a US number. The first time I evidently got in just as they were opening up; the second time the available hours were set to GMT, and worked out to 3:00 am to 3:00 pm my time (Central Daylight.) I called about 7:30 my time, and was only on hold about 2 minutes.

Update. Just got off the phone with WestJet. It was explained to me that the computer initially notices when a flight is cancelled and tries to find the best solution. It reads each leg of the trip separately and looks for solutions and since it could still get me from Calgary to Halifax it just changed that one instead of cancelling it. So while I was on the call, the agent cancelled the entire flight and as per a ruling by the Canadian Transport Agency that flights originating from Canada can be vouchered instead of refunded, the money was put into my WestJet account and I have 2 years to use it. It should be noted that the 2 years is to book the flight and the actual flight can be after that 2 year period. I'm OK with the voucher as I fly enough that I know I'll use it. Worst case scenario is that I use the voucher to book a flight for business and then my employer will reimburse me.

Good news, Allan.

Just FYI for other Canadians the CTA voucher thing is a recommendation, not a change to the law as airlines have been presenting it. If the voucher works for you, great. https://beta.ctvnews.ca/national/business/2020/4/23/1_4908925.html I am waiting to cancel our June flights, but I know it's coming. :(