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It seems to just happen when I open the Phone app on my phone during a call on my watch.

Make sure you have handoff enabled.
 
I think there is no need to "transfer" a call. If you tap on the green phone icon in the status bar on your phone, you should be able to continue your phone call on your iPhone.
 
Thanks, I ask because I'm a Wheelchair user so I'd have to put the caller on hold a second or two because I may be slow getting my iPhone out, whereas it's much quicker to use my AW and ask the caller to hang on a sec.
 
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