Hi,
it’s not what design you use, but rather the speech codec you agree to use.
When using AMR FR codec with low speech bitrate, you can have a very high protection to CIR. It can go down to a C/I of 1dB while maintaining the communication. This requires AMR FR 4.75kbps.
Using the typical 12.2kbps, the call will start dropping at C/I around 6dB.
Precise values can be found in 3GPP docs 😉
Regards
pix