I'm not at all familiar with this phone system but perhaps someone can help me. Is there a way to have all outbound calls show the same number on outbound Caller ID? What's required?

Are you using a PRI on the STS system? If so it is possible to change the outbound DID per station if it was changed to begin with.

If you talking about POTS lines that would be a question for your carrier.

I have no idea what PRI, DID or POTS are.

You probably don't have a PRI (I'm sure you'd know if you did). You probably have POTS (Plain Old Telephone Service) lines.

To reaffirm what Oobie said: The only way this can be manipulated is if you can negotiate this with the service provider for your dial tone. No phone system has the capability to do this otherwise. Sorry...

What you want is called "Caller Id Spoofing" All lines show the main number on outbound caller id. Most dialtone providers can do this upon request. There may be a one time order charge to do so.

What is a PRI?
I've talked to the carrier (QWEST) and they said there's nothing they can do, but that it can be done through a PBX system, through a software over-ride. But I don't know whether the STS system has this capability or how to activate it.

PRI is Primary Rate Interface. Think of it as a T1 on steroids. If you don't have one, you would need to both upgrade your Qwest service plus install the STS PRI board. This is not a do it yourself project for someone unfamiliar with both PRI delivery and the STS phone system.