call transfer in pjsua2

I have a problem with call transfer using pjsua2 api. Actually I don't understand how this should be implemented for call transfer (REFER method).

My issue is on the transferee: when the transferee receives the REFER message, after sending NOTIFY to the transferor it creates the call to the transfer target: But the same Call class instance is used for both calls (the lookup method changes the id to match the searched id), while pjsua has 2 different call ids for the old and new calls. Therefore, when the transferee receives the BYE from the transferor, it deletes the Call instance which is used for both calls, whereas pjsua still keeps a reference to the new call with the target transfer, which ends with a program exception.

I implemented the onCallTransferRequest() callback in the transferee but I don't see what to do here (pjsua doesn't do anything in its similar callback...) My question is: how should I process this kind of transfer using pjsua2?

Thanks for your help and merry Christmas. Thibault