Both Nokia S40 and S60 devices uses what Nokia calls Speaker Independent Name Dialing or SIND for Voice Activated Dialing. What it does, is that instead of recording your own voice tag for each or some of your contacts (older S40 was limited to 10 voice tag only), the phone now assigns or generates the "voice tags" automatically. There is no need for you to record any voice tag, and you can not do it either even if you want it! In effect, now all of your Contacts has a "Voice Tag" generated by your phone, and this voice tag is exactly as how is your contact was written. Sort of err, "Verbatim", if I may use that for lack of better words.
Many people are a bit confused about this and others find SIND ineffective. Base on my experience there are some key things that you can use to make SIND work more effectively for you. By the way, this will apply to both S40 and S60 phones.
1) LET THE PHONE TEACH YOU
Listen as to how your phone's voice is pronouncing the built-in voice tags and try to mimic it. Try to do this with the contacts the you always call, say the top 10 or so. Your family members for sure, and so forth. I would say, depending on how much free time you have, listen as how your phone pronounce the names, the more the better.
2) TEACH THE PHONE
Do the voice dial as much as you can. The more that you do it, the more that your phone will also adapt to your voice. Yes, your phone will also try to adapt to your voice.
3) BE CAREFUL HOW YOU WRITE THE NAMES
If you have not noticed it, if you type the names in all capital letters, like "ABN-AMRO" your phone will pronounce this contacts by letter, as in "A", "B", and so forth. Your phone will pronounce it "A-B-N A-M-R-O" and it will not pronounce it as A-B-N- Amro. Hence, you can actually experiment on how you are writing the names but still the key is how the phone says it, and you must mimic it as must as you can.
Hope this helps, SIND might not be perfect, but once you got the hang of it, it will work! Like in Sports, the key is practice.