Hey all Samsung certified tech 9 years now however left the PBX business in 2006 and now work for a CLEC and haven't really touched a Samsung since. Anyway have a DCS 50 si ext 206 off the main board wont ring on incoming calls. All co lines ring to group 500 and 206 is definitely a member in 500. Ringing volume is ok checked that. Also all 4 co lines have appearances on the first 4 programmable buttons on phone checked button programming. I have tried a different phone that I know rings and still no ring. The light on the buttons lights up but no ring. On a side note the customers APC ups died about 3 months ago and this started after this. Customer just told me about the issue. Maybe bad port on main board? not sure?