The word my can be pronounced the same as the word me. In England, the pronunciation is most common in northern England, the Midlands and working-class London speech such as Cockney. However, the pronunciation is also heard in Wales, Scotland, Ireland, Northern Ireland and parts of Australia and New Zealand.
The pronunciation of my with the same pronunciation of me only occurs when the possessive adjective is unstressed. In the question, Where's my phone?, the pronunciation of me is possible. However, this is not the case in the sentence It's my phone, not yours. The reason is that in this sentence the word my is stressed.
In England, the alternative pronunciation of my is common in the north, the Midlands and working-class London speech. It is common in all of the United Kingdom, Ireland and parts of Australia and New Zealand. However, the alternative pronunciation of my is only used when the word is unstressed. It is associated with casual speech.