When already on an active call, the phone is unable to toggle between calls. A 2nd arriving call hides the softkeys which were available on the 1st call. You cannot transfer the 1st call before taking the 2nd one since the only option available on the LCD is "answer"/"ignore" pertaining to the 2nd call. The only option is to answer the 2nd call, which automatically puts the 1st call on hold.
I have tried working with the Programmable softkeys by including "toggle" softkey, this also does not work.
Without seeing the phone config, it's a guess as to what's causing it. But try toggling the 'line navigation' setting to the other setting. There's only two setting for it. This parameter is in the Phone tab under Miscellaneous Line Key Settings.
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