How do I transfer a ringtone file from my MacBook Pro to a Doro DFC-0380 phone?

have you tried their guides?

Doro advice

scroll to section "Can I browse and transfer files between my Doro device and a Mac?"

and unfold it to get their step by step guide

Custom Ringtone on a Doro 7050 Phone. : r/techsupport

manual_doro_6820_en_v10_r20091.pdf

more

macbook transfer ringtone to Doro DFC-0380 phone - Google-

There has never been an Apple way of transferring that type of content from an iPhone to anything else. You will have to use a third-party tool to transfer the file, probably to an intermediate media, then use whatever tool the Doro uses to add ringtones to it. Can it even use AAC ringtones? You may have to convert the ringtone to another audio codec.

You also may find different/better answers on the iPhone forum. Note that you will likely not be able to transfer it directly as I indicated above.

I misread your post. I thought you were trying to transfer from an iPhone to another phone.

to transfer from a MacBook, you would just need to talk to the people that make your phone to find out if they offer an ability to act like another external drive or something like that for the transfer. There may be some setting on that phone to allow it to be seen on a computer. There is also the possibility that the phones file format is unknown to macOS, and in that case you would need some sort of Mac software to recognize the device.

If you have an Apple Silicon MacBook Pro, you should get a question on whether you want to allow the device to connect in order to see it in the Finder.

Have you tried texting it to your phone?