3550 will be end of sales soon! 3560 will be the replacement, If you wish to stack then 3570 is the answer,However, what about the needs of your network?
Normaly, VoIP phones, mark the CoS value in their L2 frame.For example you might have the topologyPhone --- L2switch --- Router1If the phone is setting the CoS value you just need to trusted on the L2 Switch. Then on the Inbound Interface between L2...