Both "I was about to call you" and "I was going to call you" are absolutely correct and commonly used. 🎉
I was about to call you": This often suggests you were very close to making the call, like you had already picked up the phone or were just about to dial. It implies a strong sense of immediacy.
"I was going to call you": This is a more general way to say you had the intention or plan to call. It doesn't necessarily mean you were seconds away from doing it, just that it was on your mind as something you intended to do.
Both are super natural! You can use whichever one feels more comfortable to you.