Sometimes no audio with Speedtouch 780WL

[Tribubu]

Gentlemen,
For a couple of days ago I've changed my VoIP config from a Siemens IP Gigaset to a Speedtouch 780WL ADSL modem/router and a common DECT phone. The Speedtouch has build in ATA functionality and two phones can be connected via RJ11 (FXS) voice ports. Furtermore a PSTN landline can be connected, but I don't have one. With the Gigaset I never had any problem using Voxalot but now with the Speedtouch most of the times I have audio problems. The number is dialed correctly but then the line is silent. Phone is ringing normal at the caller side but they can't hear me and I can't hear them. VoIP account details can be entered via the webinterface of the Speedtouch but these are very limited; only username, password, reg/proxy port and server can be entered. A stun can not be entered, maybe via telnet but I've not tried this sofar. I'm using proxy01.eu1.voxalot.com for now, to overcome the Betamax FUP issue but have tried the us servers as well without any luck. I've been using reg/proxy port 5060, 4060, 3060 and 80 but no improvement or difference. Ports are open and requests to these ports appear in the Speedtouch log. Beneath a log entry of a problematic call (port = 4060):

FIREWALL replay check (1 of 1538): Protocol: ICMP Src ip: my_ip_adress Dst ip: 85.17.19.194 Type: Destination Unreachable Code: Port Unreacheable

IDS dos parser : udp flood (1 of 2) : 85.17.19.194 my_ip_adress 0200 UDP 12630->48132

I've now changed the registrar/proxy port from 4060 to 5060 and everything seems to be normal again. However, I've been using 5060 before and the problem was the same then...

What could be causing these audio problems...?

Any help appreciated, regards, Tribubu