This is my telephone: https://www.gigaset.com/it_it/gigaset-c570-system-it/
I have Fastweb as ISP and these are its voip settings:
3. Requirements and settings related to VoIP service
SIP support (RFC 3261)
DNS query support for address resolution of Fastweb SIP network access proxy
SIP proxy: voip.fastwebnet.it
Proxy SIP Port: 5060
SIP Transport Protocol: UDP
Support for the PRACK method (RFC 3262)
Support OPTIONS method
REGISTER expires time: 3600 s
INVITE Expires: set to a value greater than 430 s
Supported codecs: G.729 G.729A (including version A, versions B and AB NOT supported), G.711 A-law
VAD (Voice Activity Detection): disabled
Packetization time: 20ms
DTMF tone support: in-band telephone-event (RFC 2833)
Fax protocol: only T.38
IP traffic QoS marking:
SIP and RTP: DSCP CS5
Data: DSCP 0