Further Telkom frustration

For more than a month now, I have been trying to phone my elderly Sister in Salt Rock, from here in Germiston, but all I get is number engaged sound. I then call her on my cell phone and am told that her phone is not in use, so she phones me back on the telkom phone, and I get the call with no problem. Sensing a line problem, I aske two friends, one in Benoni and one in Douglasdale, to phone my Sisters number. Both report it being engaged, when it is obviously not. A fault report from Germiston said there was a fault on the Salt Rock line. A report from Salt Rock said the fault was our instrument. So far, no one appears to be capable of finding the problem, although I suspect that there is no will to do anything about it. She can phone me, but we can't phone her. We know that her phone from here is always engaged when we know it is not. So, what now? Frustration unlimited.