They could mean the same thing. I'm just calling = definitely now. I just called = could be now / could be explaining why you called in the past (because bathe person saw they had a missed call maybe)

They have very similar meanings (for American English in this context you could use either verb tense). Adding "yet" strongly implies that the speaker expected the person they're calling to be awake (and have woken up recently) or to be awake soon. Without any other context, I would assume this was happening in the morning.

I just called - past tense (you did it in the past) I am just calling - present tense (you are doing it in the present)