Kevin,
The answer to your question depends on what kind of phone you are connecting to the switch (7970 versus the rest). If the phone belongs to any of the 79xx series other than 7970, the power consumed per phone is 6.3W.
On the 3750-24PS, each of the 24 ports can deliver upto 15.4W of power, so you should be able to hang 24 phones to it.
On the 3750-48PS, any combination of the 24 ports can deliver 15.4W each. Or any combination of 48 ports can deliver a power average of 7.7 watts per port. So you should be well within the power budget of the switch, if you use 48 phones on the switch.
7970 phones require more power, due to their color display. They also support 802.3af as well as pre-standard PoE.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus
