Incoming call in ring group beeps on 2nd extension, can it be disabled?

We are running Stable, Asterisk 11 and We have ring-groups configured . We are using Digium D50 phones running Firmware 1_1_3_0-52460 with 3 line appearances of Primary extension (ext 145) and a 4th line appearance of Secondary extension (ext 245). The primary extension is in a (Ring-all) ring-group and the ring-group is configured to Skip Busy Agents, this works great. The issue is that when the agent is using line 4 (ext 245) they hear a beep on the line everytime a call comes in on line 1 (Ring-all). Is there a way to disable the notification beep from being heard while they are on line 4 without putting all of the secondary extensions in the Ring-all ring-group?