This portion from a document may explain why RLT values can depend on codec:
AMR provides robust codecs (e.g. AFS4,75, AFS5,9) that allow the network to operate at low C/R values and still to provide sufficient voice quality. On another side, error protection scheme for SACCH (FACCH) was designed for the network using FR and HR speech traffic that can’t cope with interference level in AMR networks. This performance imbalance between SACCH (FACCH) and lower AMR FR codec modes appears as increased number of call drops due to expiry of the Radio Link Timeout timer while speech quality is reasonably good.
The problem has been solved by introducing RLT counter for UL and DL whose initial values are separately configurable for different Service Groups within Power Control object, e.g.:
– higher value for RLT for AMR calls
– lower value for RLT for non-AMR calls