If a Polycom device rings once on an incoming call , the setting is probably set to ring splash.
After resetting a Polycom, the problem will persist. reset will not revert the changes that was made to the ringtones. the changed setting will still be saved in the configuration so while resetting it will fetch this updated configuration.
To change the ringtone setting go to:
Menu > Setting > Basic > Set ring type
check if ring splash is set and if so to change select from the different ring types. Ring Splash is designed to only ring once while the call is inbound.