Talk with others IN FRONT OF HER

if you get a call on your phone, or come across anyone you know while on a date, then talk in front of her, do not move away from her to give the impression that she is being excluded, or that you have something major to hide from her.

if it is too difficult, tell the other party that you will call them later, and put a fullstop to that right then.

believe me, one of the reasons for which i never could trust my ex-boyfriend fully was because he would get up in the middle of our conversations to receive calls and go away to talk in hushed tones.