Israel has already been fighting a war with Hezbollah that Hezbollah declared. These attacks were fairly specifically targeted at Hezbollah's military equipment. They have been arguably successful at disrupting Hezbollah's communications, and likely command and control systems. That by itself is a valid military objective.

To the extent that these attacks directly hurt Hezbollah personnel, and to the extent that they damaged Hezbollah's morale: those too are valid military objectives.

So "war crime" gets thrown around here quite a bit just because there are high civilian casualties. The facts are twofold: Civilian casualties have always been a part of warfare; and there is no specific number or proportion that makes some act into a war crime. That's just not how these kinds of laws are written.

I have not yet seen a strong argument for a specific war crime rooted in a specific basis in international law. A lot of people bring up protocols 1 and 2 to the Geneva conventions, but Israel and the US have not ratified those.

There are other conventions that regulate weapons of war, but I'm pretty sure none of them are going to address pager bombs directly. An argument there would have to be at least somewhat creative.