Introduced in version 4.4.0, snom and polycom phones will ring slightly differently for calls that are from internal or external sources. The incoming SIP messages are modified to include an "Alert-info" header.
The enable, disable or change any of the settings, go to the Proxy settings of your System found in
System -> Servers -> (your server name) -> SIP Proxy
And you should see these settings:
To change the ring tones for polycom phones, look in the polycom settings:
Polycom Phone settings -> Sound Effects -> (show advanced...)
The internal ring tone defined there is only used if the proxy is set to tag internal calls.