You can configure Low Latency queueing on the WAN interface, but I suspect it won't help here - the problem is most likely the latency across the public internet. A good test is to do an extended ping from the source to the destination. Rather than the default 5 pings, do somethig like 1000 so you get a much idea of the latency between the sites. It may also show how many packets are dropped. If the pings showed excessive delay and packet loss, then no configs on the gateway router will help. It is common to see these issues with VOIP over the public internet. Some providers even block the well know H323, SIP and UDP ports that VOIP use.
Well, traffice shaping won't do you any good since you have no discardables. Serialization won't help since the two B channels are already small (64kb). Your plan raises many many questions and might not be the best suited for your situation.
Why use dial-up ISDN? (assumption unless you have always on ISDN but that's VERY remote since its an international call) You still pay long distance charges and per min rates. You really don't save anything using ISDN. One of the main purposes of VOIP is to save long distance charges. ROI is the key. A better method is to get a Fr link that is about the same price but without the long distance charges and you could get a SLA and QOS policies along with everything else.
If you have not changed the default codec (I believe it is g711), your calls will take up between 80 and 90 K. You could opt for compressed voice using g729 codec, but the voice quality drops significantly ( which is your problem in the first place). Your issues are most likely with the public internet.
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