The possibility to change your Ringtone depends on the PBX - Phone system you are using. And that is the same with a custom ringtone.
Please be aware, that the ringtone have to be in a special lowband format and have to saved on a Webserver, because of Snom Phones just use a Link to custom Ringtone.
Please see Details below.
To set the ringtone for a specific identity, change the following settings on the phone´s web interface:
If you don´t know how to get into your Snom Phone Web User Interface, please have look here: Entering Snom Deskphone Web Interface - WUI
The contacts in the address book can be set with a special group, for example you can have contact Mom in category Family:
Then you can configure the custom melody for all contacts of type Family under Preferences → Audio → Directory Ringtones:
The PBX can send the SIP Alert-Info header inside the SIP INVITE method sent to phones in order to choose a certain ringtone reachable to the phone over http (custom melodies). Here is an example of the SIP alert-info header in a call that would ring using custom melody http://downloads.snom.com/documentation/melody2.wav :
If the user has set a melody and the server is sending the Alert-Info header as well, the Alert-Info header has higher priority. This can be changed by setting the following setting to 'off': Advanced -> Audio -> Alert Info playback
The wav file should comply to the following requirements:
Note: if the resulting melody sounds weird you can try without the extra encoding by removing parameter "-acodec pcm_alaw":
ffmpeg -i <infile.(wav/mp3)> -ac 1 -ar 8000 <outfile>.alaw.wav