There are a number of contributing factors which can affect the audio performance of your device. For example:
1. If the handset has been subject to immersion in water then the microphone and speaker will need to dry out for them to resume to normal functionality.
2. If the incoming call has been routed long distance and is being routed through a number of service providers telecoms infrastructure.
3. Technical performance between handset manufacturers, this can be software or hardware related.
4. The environment the handset is used in, especially where there is background noise, typically traffic in a busy city environment or large shopping malls and airports.
All handset manufacturers must design equipment to work within very rigorous standards set by organisations like the European Union, for example, these are normally safety based so that you do not suffer from hearing problems with extended use. All CAT products meet these safety standards and hence are CE marked as an assurance that we comply with these standards and our audio levels are within safe limits.