Just got off the phone speaking to a lettings agency in Pwllheli. After introducing myself in Welsh and asking about the properties I was interested in (also in Welsh) I was put on hold whilst the lady at the other end of the phone spoke to a colleague. I also handled the courtesies of saying I was happy to wait and of resuming the call, all Yn Gymraeg.
I did slip to English to describe my current address in England (still only 90 percent confident, if that, with the Welsh Alphabet and, anyway, English place names and addresses use English letters). This (unsurprisingly) triggered a code switch to English at the other end. So I was asked my phone number in English. But then (magic!) I gave my phone number in Welsh and the agent switched back to Welsh to finish our conversation.
I guess I must have sounded sufficiently relaxed / confident in dictating the Welsh digits of my phone number to trigger the reverse code switch