Flash button on analog phones and switch hook flash

[telecomadmin12]

I have 2500-type analog phones. To make a three way conference I use the switch hook flash. When on the first call, I press the switch hook, hear stutter dial tone, dial third party and press switch hook again.
Many of my phones have a flash button but pressing it won't do the same as pressing the switch hook, I don't hear stutter dial tone and can't make a three way call. Is there any adjusting I could do to make this work?

 

[montyzummer]

I have had this and found it down to individual manufacture , sometimes the flash key works sometimes it does not , not really any help , but sometimes there is a switch on the bottom of the sets hidden under a tab that will allow you to change the m/s of the flash key.

APSS (SME)
ACSS (SME)
ACIS (UC)