Dialer
Dialer was a small, single-purpose app I made for my spouse, that would let her place calls from her cell personal mobile phone but have the Caller ID display the number of the health clinic where she was working.
Patients would call the clinic phone number, which was answered after-hours by an answering service. The answering service would log the call, take a message from the patient, get a callback number, then forward a message to the physician on call. The physcian would receive the message, perhaps do a little preparation, then call the patient back. The patient would answer the call and get to talk to the physician about their problem.
If the patient had another question, they were supposed to repeat the steps above, but often-as-not, they'd try calling the physician back directly. (Why bother leaving a message and waiting for a call back from the physician when you could just call the physician?). Unknowingly to the patient, their call wasn't logged, as it was supposed to be, and if they were calling back the next day they weren't calling the doctor on call.
To discourage patients from calling back, the physicians would turn off Caller ID, so the patient would see a call from Unknown. But this meant that most of the time the patient wouldn't answer.
This is where Dialer came in: Instead of calling the patient directly from a mobile phone, the app would place a call from a calling service, and would set the caller ID with the name and number of the medical clinic.
This worked remarkably well: The patient, expecting a call from the clinic, would always pick up. Furthermore, if they wanted to call again their call history would have the number of the clinic, not of the physician or Unknown.