Some PABXs allow using FLASH event which makes it easier to plan numbering as special PABX function and "regular" numbers can be completely separated. FLASH event in SIP may be not widely known. There are few ways to transmit it:

tSIP uses RTP (RFC2833) events for signaling. By convention FLASH event is named as "R" (*), so if you want to create FLASH button set button type to "DTMF", number to "R" and set description of your choice. FLASH character can be combined with others to call feature code directly, can be also used for transfer prefix for BLF, Presence or Speed Dial button or as part of call pickup code for BLF or presence.

(*) Side note: softphone can also send A, B, C and D signs, but since they are rarely used they were removed from main keyboard at one point to make other keyboard buttons bigger. Since small button column in main part of the window is also programable this may be good spot to place these buttons if needed. It's also worth noting that many desk phones that do not support FLASH (e.g. yealink) may have A-D keys programmed and used as FLASH substitute.

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