Can you provide SIP trace of the call on FS and ZyXEL? That may provide some details of the problem.
On FS, no prob, a bit harder on the ZyXEL, as I don't have access to the outgoing (ADSL) side of it.
I remembered a similar problem i faced with an ITSP in Pakistan, and after tracing problem for a couple days i found that they were using User Agent filter to allow calls only from their "preferred" devices. So, we had to hack that to get it working with FS,
Ouch! Nasty! But in the case of telfort.nl, they do support 3rd party hardware. This is what their guide says for the settings: SIP proxy: sip.telefoniedienst.nl SIP service location: sip.telefoniedienst.nl SIP signaling port: UDP 5060 SIP register service location: tel.telefoniedienst.nl SIP register signaling port: UDP 5060 SIP domain: tel.telefoniedienst.nl URL type coding: SIP Registration interval: 6000 seconds Session expires: 1800 seconds Reserved RTP port range: UDP 16384-32767 Codecs: First negotiated G.729 secondly negotiated G.711 a-law type DTMF mode: PCM Sending INVITE Anonymous: Not permitted Echo Cancellation G.168: In use Voice Activation Detection: Not in use SIP SUBSCRIBES messages: Not permitted Early Media acknowledge: Not in Use Julf