Calling out is typically controlled by COR (Class or Restriction) and Tenant, if they are the same on both phones and one cannot call out, you are most likely using Outgoing Call Restriction... From the console access the Guest Room menu and enter the extension number (even if it is not a guest room), you can enter the outgoing access the extension is allowed there, even for admin phones.
Calls ringing in are controlled by a variety of means, mostly depending on the type of trunks... Do you know if this is LS/GS trunks (traditional dialtone) or a T1/PRI (High capacity digital trunking)?