The controlling factors should be the radio conditions.
When this feature is activated, the eNodeB monitors radio network conditions, and periodically sends optimal uplink physical bit rate to the UEs which support RAN-assisted codec adaptation specified in 3GPP Release 14.
In poor radio conditions, a lower bit rate is recommended, and if it is accepted by the UE, the lower codec rate results in fewer dropped packets through the call.
In good radio conditions higher codec rate is proposed, and the UE adapts to the recommendation, if this is supported.
E2E volte calls are either on EVS or AMR. The type of codec is negotiated only once during call setup. This is done in two steps. In the first step the caller includes information about supported codecs in the SIP INVITE message. At the end of this message all supported codecs are enlisted. The callee selects one of the codecs and informs the caller about the chosen codec in a 183 SESSION PROGRESS message. So codec type is selected at this phase ensuring both UEs support the same codec.