The screenshot below shows the configuration for a Optipoint 410 standard SIP device:
The default administration password is 123456.
This phone has some problems of keeping alive the UDP connection for SIP datagraphs. There is an option called "NAT keep alive" which is supposed to send keepalive packets every few seconds which unfortunately does not work. Alternatively the phone can be configured to re-register every 120 seconds by setting "Registration timer value" on the SIP environment page to 120 seconds.
By default "NAT keep alive" is set to "0". It is important to set it to a suitable value.
Otherwise you won't receive incomming calls.