I agree with the above statements. :)
a. I called you yesterday, but you didn't answer.
b. I called yesterday, but you didn't answer.
If you called the person directly (on their phone), either of the above would be acceptable in the US. :)
** I have to note here that saying "but you didn't answer" could be interpreted by the recipient that you are upset with them, accusing them of ignoring your call, etc. Therefore, I am including some alternatives below. :)
Common polite SPOKEN versions used in the US:
a. I tried calling you yesterday, but I couldn't get a hold of you.
b. I called you yesterday. You didn't answer, so I left a voicemail. Did you get it?
c. I called you yesterday and left a voicemail. Did you get a chance to listen to it?
Hope this was helpful. :)