First, find a safe place to wait.
A door that clicks shut behind you can turn an ordinary trip outside into a problem. If you can, stay somewhere well lit and away from traffic while you make the call. If a child, person or animal is in immediate danger, contact emergency services first.
Tell us whether you are outside a house, an apartment building, a business or a vehicle. A shared entrance near Davis Square is different from the door to your own apartment. Knowing which one is locked helps clarify the next step.
A few details make the call more useful.
Have the street address and unit number ready. Say whether a key is inside, lost, broken or no longer turning. If you can safely see the lock, describe whether it has a keyhole, a keypad or a separate deadbolt. You do not need to name the brand or mechanism.
For an apartment or managed building, a landlord, building manager or authorized keyholder may have a working key or need to approve access. Have identification and a way to establish your right to enter. The required verification depends on the situation.
Entry and changing the keys are different jobs.
Getting back inside does not necessarily stop a missing key from working. If your keys are lost rather than simply indoors, discuss rekeying suitable locks after the access problem is assessed. Shared and master-keyed doors need permission before changes.
Ask about the current availability and the estimated charges before arranging a visit. The lock condition and access method matter. An inspection may change the scope, so agree on any additional work before it starts.
A useful question
Will opening the door mean replacing the lock?
That depends on the hardware and the condition of the door and lock. Describe what happened, then ask what approach is proposed and what any replacement would cost before approving it.