My Nokia E71 running Symbian S60 already has SIP capabilities built in to the actual OS by default – it works perfectly.
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Bonded ADSL
It’s probably worth opening the Task Manager and going to the Processes tab and order by CPU usage – see if Yahoo is using 100% CPU, or 50% if you have 2 cores etc. Could explain the rest of the system being slow.
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Bonded ADSL