Hi,
The Catalyst 3550 switch supports only Cisco's proprietry version of power over Ethernet as it was released prior to the IEEE 802.3af PoE standard being ratified.
The 7942G Data Sheet and Q&A does state that the 7942G is compatible with Cisco pre-standard switches so in theory you should be ok.
Data Sheet
Supports IEEE 802.3af PoE (Class 2). Also supports Cisco Prestandard Power over Ethernet (PoE), allowing powering from any of the Cisco Inline Power-capable blades and boxes, plus Cisco midspan. 48VDC is required; it can be supplied locally at the desktop using an optional AC-to-DC power supply (part number CP-PWR-CUBE-3=) or power injector (CP-PWR-INJ=).
Q&A
Power
Q. Do the Cisco Unified IP Phone 7962G and Cisco Unified IP Phone 7942G support Cisco Inline Power?
A. Yes. Additionally, the phones support IEEE 802.3af Power over Ethernet (PoE) (Class 2) and Cisco Midspan Power.
Q. As Class 2 IEEE devices, do these phones require a continuous draw of 7 watts (standard Class 2 allocation)?
A. No. When Cisco Unified IP Phones are connected to common switches such as the Cisco Catalyst® 3560 Series Switches or Cisco Catalyst 3570 Series Switches, the switch can automatically detect the phone type and provide the exact amount of power that the phone needs. The use of Cisco Discovery Protocol provides this greater granularity to the power classification, allowing more accurate reporting of power consumption. The Cisco Unified IP Phone 7962G and 7942G models draw a maximum of 6.3W from a switch.
What type of issues are you seeing? - can the switch power a certain number of phones or does it have problems if just one phone is plugged in?
You could try issuing the command below which seems to have solved issues for other users according to this
thread
power inline delay shutdown 20 initial 300
