Hi,
The "Auto Complete Number" was introduced as a feature in Cisco 7960/40 model phones, where you can turn ON/OFF this feature from the instrument. This options looks to be added into the Firmware now, which by default is available in all the later introduced phones and not listed anymore as a advanced call handling feature.
So you cannot turn off this option in 8941 model IP Phone. If you feel it is a critical issue, either change the way dial plan is used or raise a firmware level feature request for ON/OFF this feature from Instrument.
HTH,
Gifton Abel.