I hope she surfaces safe and well soon. I agree if she'd gone to the airport and you were not there she'd have contacted you by now. How old is she? Is there any chance she's being held by family to be forcibly married off? I don't know enough about the culture in Cameroon to know whether this is likely or not.
It's possible (I hate to say this, but it is...) that she is in on the scam and the phone going dead and leaving you wondering, then no further contact since has been done purposely to ramp up the fear in you and make you think there is real danger for her.
Perhaps they will contact you again later with a terrible story of kidnap or arrest or something and then there will be requests for money.
And of course it's also possible that it's all true and she really is in danger.
The fact that you don't seem to know her well at all and yet you were offering (or responding to a request) to pick her up from the airport makes me wonder if you've been targeted as a bit of a soft touch and Patience is actually going to try to rinse you.
I'm sorry to say that but I know several people it's happened to when they've befriended (or started romantic relationships with) people from poor countries. Before you know it, you are regaled with all sorts of drama and sob stories about terminally ill mothers needing money for hospital care etc., storms blowing the roof off the house, sister dies and leaves four babies with no support etc., and your conscience gets the better of you. You become fair game and a meal ticket. A means to an end.
That said, she's a British citizen with a job here, so it doesn't really make sense or fit the pattern.
Apologies to Patience if I am doing her down but you do have to think rationally about the likelihood of all this being true when it is also bears all the hallmarks of a scam.
All you can do is go down the official routes and hope to hear something positive soon. She may just have decided to not come back just yet and didn't think to tell you. Which is rude, but not beyond the realms of possibility.