Many VoIP services provide a better quality voice transmission than many would be expected from mobile calls, and some can even provide a better quality than calls made on the regular fixed home line. Some VoIP services can offer an enhanced audio quality that is not available over the PSTN phone line.
However, VoIP services may be subject to more variation in the quality of calls than calls that are made over a PSTN line. This variation may be caused by congestion elsewhere on the internet.
Calls made over a broadband connection are subject to variable performance due to competition for resources on the connection, for example, if the broadband connection is being used to download a large file while a VoIP call is in progress or where the connection meets the public internet.
Check with VoIP providers about the information they can provide on the quality of their service.
The audio quality of your equipment may also factor in the clarity of audio you receive over VoIP. If headphones or microphones are faulty, it will make it difficult to talk or be heard.