The simplest thing would be to move the SIM card back into Nexus S, note and write down the VM number, then move it back into Nexus, bring up the phone app, go into Settings, then Voicemail settings. You should have at least one entry under Voicemail service (probably called My Carrier), then go to Settings for My Carrier and specify the voicemail number. BTW, it may pick it up from the SIM as you're poking around those menus.