it is frequent for a DXer to make a phone contact and ask the DX
to change to CW (on the same phone frequency) to confirm that mode.
Perfectly acceptable under the rules and is common.

Werl, I dunno about 'common' but I've done it once, in order
to work Tromelin on CW for the first time. And I didn't even
have to ask... FR5ZQ/T had a big 'phone pile-up in the 'phone
part of 20m and I knew that he worked (and preferred) CW - he
was also my first FR/G on CW. I sent my call in CW over the
'phone pile-up and he came right back on that mode. I didn't
wait around to hear what happened after that, but I imagine
that some of the callers started opening drawers looking for
their Morse keys...

Derek
aa5bt