There can be two reasons for this: The conference guide's settings. Check or update the following: 
1. Click on Menu 
2. Click on Conference Guide and then choose Settings 
3. Check/update the setting for request, return and conference. Different PBXs and countries have different flash times You must have the right settings for the conference guide to function properly. 

Check or update the following: 
1. Click on Menu 
2. Click on Setting and then choose Flash/R 3. 

The predefined Flash time is highlighted. 

Below are the recommended settings. 

Flash times: 

100 ms – Scandinavia, Europe, Siemens Enterprise Communications HiPath 

300 ms – Germany, France, and Israel 

500 ms – USA.