Which vendor are you working with ?
SD Drop shouldn’t be linked to AMR, unless perhaps the following situation :
AMR codec is required during TCH assignment, while the MS is sitting on its SDCCH.
The TC and MSC must allocate Ater and A interfaces channels to carry the TCH. And the TC should be able to transcode the AMR codec into the G.xxx codec. Perhaps the TC is not able to serve all the AMR calls in the BSC ? Therefore some calls fail ?
Regarding the intracell HO:
AMR is a great feature that allows rate change during the call (within the same cell). For instance, FR to HR or HR to FR. This is done thx to intracell HO.