Can't dial out

My phone won't dial out anymore. The only thing that is appearing is the indicator arrow next to the "intercom" button. It will not select a line or dial out. But there is a dial tone, and it will accept incoming calls.
There were some guys in the office doing some work earlier today. Could they have hit a combination of numbers that would do this? Or are there buttons that are jammed?

The phone is a Nortel Norstar Modular DR5 with three lines.

Thanks for your help.