Hi Mike,
I am a PABX/Voice engineer and use VOIP all the time and ADSL1, especially at that speed, isnt the best when it comes to VOIP calls and keeping some bandwidth available for internet use. Like Warren and Eric suggest, it works for some, but my experience is no two ADSL links are the same, even faster ones sometimes dont perform when you put VOIP on them.
It could be that the Oricom router/phone is set to use a high bandwidth compressor like G711 (which uses 100k/100k per call). Check the compression algorithm the tools options or the configuration menu and set it to something more efficient like G729 or G723.1 if they are available and / or supported. This should bring the bandwidth per call down to a more acceptable 30K per call.
This may help. Also, it could be there are Quality of Service (QOS) settings in there which effectively prioritise the voice traffic over the data traffic and these are probably doing their job as voice needs to be realtime and data doesn't.
As for the 1500/256 link speeds, check this at
www.speedtest.net whcih will measure your ACTUAL throughput speed and you might find that you could only be running at 1.1M/128K for example, which means ANY voice call will have a significant impact on data speeds.
Check with your internet provider (and maybe do a competitor's estimated speed check on your phone line from their websites) to see if you can be upgraded to ADSL2+ speeds if its available on your exchange as this is MUCH better with coping with voice. You will probably also find the price would be similar to the price you are currently paying for ADSL 1.
The whirlpool forums will also probably have a similar situation to yours answered there, so check these out for any other tips on your setup.
Good luck, hope you can sort it out.
Cheers
Chris