Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do what I can to provide the right answer to your question.
I suspect the problem is not in your Gigaset phone, but rather in either your phone line or your phone system box.
I don't know the context (office with many lines, versus office with one dedicated line, versus home with several lines, versus home with only one line, VoIP versus landline, FIOS or UVerse or Comcast etc...) so please tell me more about the problem.
If you have no phone system box, this might help:
1. unplug the Gigaset phone from the power.
2. check whether it is plugged into the phone line properly - remove the phone cord from its jack, dust out if necessary by blowing, check the end of the phone line cord, and put the phone line cord back into its place in the Gigaset phone.
3. wait at least 10 minutes, with the phone still unplugged from the power. (Note that this means the base station as well as the handsets, but not the phone system box if you have one... just the Gigaset phone.)
4. plug in again to the power, wait 1 minute, then try making calls (-Can you call out OK? and can you now receive calls properly?)
The problem - barring your having a phone system box in the works - is perhaps that the built-in answering machine in the Gigaset phone is malfunctioning and half-picking-up on incoming calls, but not really answering them either. That's a guess, but the above might address the problem... and also other measures to try, if the above does not work or is inappropriate according to the actual full context of the problem. So let me know more about the problem, please. Thanks.