Please provide more information about the hardware or VoIP service you are using.
If you are using a regular Voice Modem (connecting to 1 phone-line), then of course you will be only allowed to make 1 phone call at a time. It wouldn't be any different than using a regular phone. If however you purchase a telephony board (that connects to multiple lines) and you hook it up to several different phone-lines from your telephone company, then you will be able to make several calls at once.
With VoIP, the number of simultaneous calls you can make is limited by the VoIP provider. A lot of providers allow for two simultaneous calls (as this allows for Call Waiting). Some of the cheaper ones however only offer 1 'line. Other providers, especially those for businesses may offer additional communication channels, and other providers choose to charge a certain amount for each channel.
