Can the concentration of a sodium carbonate solution be determined using volumetric analysis?
The titration of hydrochloric acid with sodium carbonate is a common school practical, used to determine the concentration of the acid (standardisation). 

In the first step, as you rightly point out, the sodium carbonate reacts with one equivalent of hydrochloric acid. The phenolphthalein shows the pH change associated with the formation of the sodium hydrogen carbonate (aka bicarb). You could use this value to calculate concentration, however, in order to ensure your answer was correct, you'd need to run this titration at least twice more, averaging your answers and discounting any anomalous titres. 

$\ce{Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)}$

As the bicarbonate is also reactive with hydrochloric acid, you can do something known as a 'double titration'. After the first titration, you can immediately do a second titration, this time using a different indicator, such as methyl orange. By doing this, you get a second value. If your titration is good, the values for the first and second titrations should be equivalent. 

$\ce{NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) }$