Can sodium carbonate-sodium bicarbonate buffer be used for EDTA titration of calcium?
Sodium hydroxide $(\ce{KOH})$ works as $\mathrm{pH}$ modifier (measure the $\mathrm{pH}$ by adding drop by drop of hydroxide until $\mathrm{pH}$ is approx. $10$ to $12).$

Carbonate ion will be combined with calcium(2+) cation to form calcium carbonate. You can measure the calcium content by precipitating the calcium using soda ash light solution. Good luck.