tSIP: call transfer
Up to version 0.2.x tSIP supported directly only blind (unattended) call transfer. Attended transfer with REFER requires handling two simultaneous SIP calls. This complication (affecting code, plugin interface, probably also user interface) does not seemed 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.
Unfortunately not every PABX (e.g. FRITZ!Box) and not every operator support attended transfer using feature codes.
Support for multiple calls along with attended transfer is planned for tSIP 0.3, currently available as a pre-release build. To use it you have first to define at least two LINE buttons (enabling at least two concurrent calls) and define "Attended transfer" button. After establishing first call you can start second one switching to other LINE and then use "Attended transfer" button.
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