"Yes, you must phone her."
Translation:Oes, mae rhaid i ti ffonio hi.
The sentence doesn't necessarily need it. "Oes, mae rhaid i ti ei ffonio (hi)." (with the "hi" almost always being included otherwise it could also mean "You must call him" as well), would be considered the more correct way, at least it is what is traditionally correct. However "Oes, mae rhaid it ti ffonio hi." is now a common enough, and acceptable enough alternative that both are fine. You simply have been shown the version without "ei" due to Duo's system.