Of course setting ringtones is quite easy on any Android phone:
You simply open Settings, select Sound and then select Phone Ringtone and add one from the list.
But what if you want to choose an MP3 file stored on your phone memory or on your SD card as your phone ringtone ???
Here I will explain two ways to achieve this easily.
NOTE:
Both the way that I am going to represent, require a file manager application.
By default file managers come already installed on devices.
But still I recommend a free file manager application to be installed by you if you are not comfortable with the installed file manager application:
File manager App from Play Store
The easiest process:
STEP 1:
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Open Settings, select Sound and then select Phone Ringtone
[Goto Settings, Sound, Phone Ringtone]
STEP 2:
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Select the File Manager application
[Select the File Manager application]
STEP 3:
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Go to the MP3 file where it resides in your phone memory or in your SD card memory and slect the file.
After this step you can see that your Ringtone is set to the MP3 file that you had selected.
[Select the MP3 for Ringtone]
Second process:
STEP 1:
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Open the file manager.
[Open the File Manager application]
STEP 2:
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Create a folder in the name of Ringtones inside the sdcard or phone memory.
[Create a new Folder and name it as Ringtones]
STEP 3:
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Go to the MP3 file where it resides in your phone memory or in your SD card memory.
[Browse the MP3]
STEP 4:
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Then copy the MP3 file and paste it in the Ringtones folder you had created in the second step.
[Copy and paste the MP3]
STEP 5:
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Open Settings, select Sound and then select Phone Ringtone and search the ringtone that you have kept with it's file name and select that from the list.
[Your MP3 under Ringtones]
I believe the first step is quite easy than the second step.