There are 3 ways to set up distinct ring tones on Yealink phones.

The first would be to have 2 accounts registered to 2 different extensions on each phone, with each account set to play a different ring tone (this can be done by going to Account > Basic and changing the Ring type). We then set the DIDs to point at the different extensions. 
The second way would be to add the numbers you wish to have a different ring tone to the phone's local directory where you can then assign a different ring tone for each number. You would do this by:

• Getting the IP address of the handset by pressing 'OK' to bring up the status screen
• Entering that IP address into your browser then logging in with 'admin'/'admin'
• Go to Account > Advanced and ensure Distinctive Ring Tones is enabled
• Go to Directory > Local Directory
• Under Group Settings, name and set a ring type for the group then click 'Add'
• Under Contacts, Enter a name (such as line 1) then the phone number that you want to hear a different ring tone for
• Set the Group to be the one you just created
• Hit Add
• Repeat the above 3 steps for every number you wish to add. 
  The Ring option in group settings allows you to set a different ring tone for all numbers in that group, you can also set it on a per number basis with the ring tone option under Contact.

The third would be to set the PBX to send a SIP alert in the Invite to the handset and set the handset to play a different ring tone via the Internal Ringer text in Settings > Ring.