I have a 2821 rtr serving up ip addresses to some 7961 phones all connected to a 3560-48 port switch and everything has been working great.
The other day the 3560-48 power module went out so no POE.
We can get the phones online with local power and the 3560-48 which is great.
However I decided to replace the 3560-48 with a 3560-24 from our offline stock instead of installing another 3560-48 since the 3560-48 is way more than this site needs.
The 3560-24 loaded up and when it is installed it comes online just fine.
I can telnet to it, ping from it to anywhere in our network, and it delivers POE to the phones.
Other than having fewer ports on the 3560-24 the config is the same and the IOS is the same as the 3560-48.
However the 7961 phones will not pickup an IP address when the 3560-24 is online.
When the 3560-48 is installed again and the phones are plugged into local power they pickup an address from the router just fine.