If after checking this the issue is still not resolved then this sounds like this is an issue with the line identity that the phone handset is associated to.
In order to check this we may need to log into the Gigaset N300/N510 Base Station and check which line identity is used for outgoing calls to make sure that the correct phone handset is associated with the correct phone line and then it should work correctly.
To read more about the 'Handset number assignment' settings please click here.
Does the issue only affect calls that are made manually instead of using Click2call?
For example if you have your Click2call correctly set up can you please dial via Click2call to check the line association is correct?
If you then manually enter the phone number using the phones keypad and the outgoing call does not associate with your line and user profile correctly then please refer to the 'Handset number assignment' settings as by correctly associating your handset with the correct outgoing line this should then resolve the issue.