There is a one hit ringing option for stations that are set to call forward all. Aside from that more testing and more information is needed to advance.
1. Are 509 and 510 new extensions?
2. If so, is 5XX valid number plan?
3. Have you tried canceling all FWD and DND options at 509+510?
4. Are 509 and 510 functioning for outgoing calls?
5. Are they IP phones? (PAD card or network issue)
6. If 509 and 510 are TDM phones on the same blade? Was that blade reset?
7. Are 509 and 510 part of an ACD?
8. When all else fails, a system reset may be required. Perform a database back up first.