Message not compatible with Protocol state

[Spider]

Hi Guys, Actually i faced many disconnects on Iu interface with cause "Message not compatible with Protocol state" sent from the CN toward the RNC. After deep analysis, i observed that these rejections mainly happened after a Call (OC or TC) with Succesful relocation from 3G/2G than after normal clearing the user tries to make another call within few seconds (20-60 sec) and he faced directly this reject. I assume that after the relocation the user remains in 2G , so how we will be able to make a call from 3G? and after the relocations what are the procedure involved and their related times?

Thanks in advance