Technically the answer is 'yes' and there are customers doing so. You would need to use the MyNetFone soft-phone client on your PC with G.729 compression. This will compress the voice small enough to fit into a dial-up connection.
However customers should note that voice quality over a dial up internet connection will vary greatly with the quality of the service and the modem connection speed. MyNetFone does not recommend using VoIP with a dial-up connection and does not provide support or refunds to customers using our service over a dial-up connection.