Can you give any further information like manufacturer and model of equipment?  

Or power draw? Voltage-Current?? Anything?

We use an APC Smart-UPS 1500VA on our phone system.

You will need to determine the max power draw of everything you want the UPS to support. Then you can use a tool like below to select a unit that meets your needs.

If you have a vendor that supports your phone system I would check with them for recommendations.

http://www.apc.com/products/family/?id=165

Tripplite and Eaton are two other vendors you might consider.

Would you think allowing 300 Watts power draw for a rack server and PABX is sufficient?

Level 1 Support Technician

I have one of these "Kill-A-Watt" power measurement gizmos:

http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU

They are all over the place so should be easy to find one.  On a weekend or whenever you can power cycle your pbx hook one of these up and measure the power draw.

We have a Toshiba CIX670 with 4 cabinets supporting around 300 phones.  It draws around 300 Watts.  If I plug 300 watts into the APC sizing tool above, I get a run time of 56 minutes.  

Servers tend to be more power hungry than a pbx so not knowing your situation, 300 watts may be a little on the low side. Just get a Kill-A-Watt and you will know exactly what the power draw is.  Just remember that the idle power consumption will probably go up when the system is busy to give yourself a little margin.

You can also get extended run time UPS's that allow connection of an additional battery if you want really long run time.  They will of course cost more and you will be spending a lot more on batteries.


 

To get an estimate for power required for your server just take the rating on the sticker of one of your server power supplies (assuming N+N) and multiply the watt rating by .65. You will need to convert the sticker rating to watts: amps x volts x .9 = watts

http://www.datacenterinfoblog.com/