US Customer in Germany

[HeywireCommunications]

I have a US-based customer that's temporarily relocated to Germany. His ISP there is TKS. He's connected his ATA (Grandstream HT-486) to the Motorola SBV5121e cable modem that TKS provided. We went through the ata's configuration and everything appears to be working well in our software, but when he receives and incoming call, the line goes dead when he picks up. We thougt it was a bad phone, so I had him disconnect it from the ata. It rings until the voicemail picks up...then goes dead. He can call out, but only at about a 30% success rate. When he calls other customers on our network, including our Tech Support, both phones hear a busy signal. I'm wondering if there's a known issue with VoIP in Europe, a known issue with TKS (he can't get any support from them - no one in Tech Support speaks English, only in the Sales department), or if anyone knows how to access the configuration of the SBV5121e so that I can see if there's a DMZ field that I can put the IP address of the ATA in.

Our system isn't showing any error messages and it looks like all is well. I know there's a solution out there, I just don't know where to look.

Thanks in advance,

Thomas Waller
Heywire Communications