DLink DPH 80S configuration
 

I configured a DLink sip phone the DPH 80S with a Voxalot account but it is not working as I would like it to work.
I'm able to call my Voxalot contacts, using their Voxalot numbers and have a conversation as long as I like, without any trouble.
When calling my PSTN contacts with the DPH80, by making use of a Betamax clone through Voxalot, there's a problem. The line drops as soon as the call is picked up from the other side.
When using for instance my PAP2T in the same way, no problems with call drops, I can call my PSTN contacts and have a conversation.

My sip configuration details in the dph80:
user name: xxxxxx (my 6 digit voxalot number)
pasw: ******** (my voxalot passw)
phone number: xxxxxx (my 6 digit voxalot number)
phone port: 5060
proxy server address: 85.17.19.194
proxy server port: 5060
domain name: voxalot.com
outbound proxy address: 85.17.19.194
expiration time (in sec): 3600

Note:
1) It is not possible to configure a stun server in a DPH80.
2) The fields proxy server address and ob proxy address can only be configured with an IP number.

Anyone any suggestions?
Thanks and regards.

A packet analyzer would show the SIP packets exchanged when the call is dropped. That may disclose the reason why the calls are being dropped.