It wouldn't be a first
https://en.m.wikipedia.org/wiki/Yahya_Ayyash
> Shin Bet agents gave him a cell phone and told him it was bugged so they could listen in on his conversations.[17] They did not tell him that it also contained 15 grams of RDX explosive
Replace half the battery with c4, you really don't need much to injure someone in direct contact with a modern explosive
I imagine technological progress has been made in the field of explosives as well. I guess the volume and weight can very very small when you can determine that the phone is being held at the head and the explosion can partially be directed to it. Maybe it's even possible to leverage the energy within the battery?
Cell phones were much bigger in 1996. You would have a hard time with that with phones today, but pagers are still viable.