By putting an EIR in your network, it forces the MS’s to send their IMEI to the network. It is done thanks to the UL message “IDREQ”. The MSC replies with the “IDRES”.
As this message is vendor’s dependent, there is no specific time when these messages shall be performed. Usually, it is done during the call setup, over the SDCCH channel.
Due to wrong configuration, maybe the IDRES takes a lot of time to reach the MS. This procedure might increase the time the user stays on the SDCCH. –> it increases the chances of SDCCH drop.
The SDCCH dimensioning is not the problem : if you had SDCCH dimensioning problems, it would lead to more SDCCH congestion. Not to more SDCCH drops.
In order to investigate the problem, you can perform an A-trace of a few call setups, and check the duration of the SDCCH phase (starting with the AUTREQ authentication request, ending with ASCMP assignment complete)