Hello Ali,
Preparation failure, during the TCH assignment ? The only experience I have about this failure was linked to a missing patch on the NSS side. It impacted heavily all cells of the network, as soon as they were congested. I think it was due to a wrong management of the Directed Retry or the Queuing.
If it only happens a little bit to few cells, I can’t say. I think nobody knows exactly why there are these failures 🙂