tSIP: call transfer
tSIP supports directly only blind (unattended) call transfer. Attended transfer with REFER requires handling two simultaneous SIP calls. So far this complication (affecting code, plugin interface, probably also user interface) does not seem to be justified as with most PABXs attended call transfer is possible with feature codes as well. For example, asterisk-based PABX would most likely used "*2" to start attended transfer, "*3" to abort it, "*5" to merge two calls into 3-way conference.
Feature codes related to transfer can be programmed as DTMF buttons as almost all visible buttons are programmable. With descriptive caption, distinct bitmap icon, optional keyboard shortcut for every button this setup should be actually easier to use than desk phone.
Feature codes can be also combined with BLF/Presence/Speed dial buttons: BLF transfer/pickup.
If your PABX that does not support transfer using DTMF codes - let me know. So far I know only of FRITZ!Box (small appliances with 10 extensions or so).
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