Please check the firmware version the phone is using - the latest version is 02.01.00 - please click here to check what version you have.
Please login to the Gigaset web interface for your phone and check your port settings which should either be '5065' or '5060' used.
Check this inside the Settings tab where you will see the Telephony - Connections page and inside the Network and Connections page - VoIP Settings that the ports used are all the same.
The phone should get a response from the PBX to say that the call has been completed (answered) elsewhere. In order to see what is happening or if the phone is getting a response we will need you to do a PCAP (Packet Capture) log.
Looking at the Gigaset DE310 user manual you can obtain a PCAP log directly from phone please click here and refer to page 112.
As part of this you will need to login to the web interface of the phone and go to the:
Status > PCAP logging > press Start > replicate the issue (ring the call group and answer the call on a different handset and then wait for the DE310 to stop ringing) > press Stop once the issue has been replicated >
Then save the log, download and send it over to us at our Support Helpdesk so that we can investigate and advise further.
Depending on the findings from the above a factory re-set may need to be done on the phone and then re-apply the line configuration settings manually rather than restoring them.