hi,
I had the same problem when I first got my Nokia N70. There is a download available from the Nokia website to fix this as it is a known "bug": I looked on the Nokia website and came across a patch to fix the problem:
Nokia say:
"Complex ring tones (MIDI, AAC, MP3 or WAV files) can sometimes take a long time to load and play. Occasionally when this happens the phone uses a default ring tone setting which may be missing. To ensure this default ring tone is present you can download this patch file and then transfer and install it on your phone. This patch is recommended even if you have not experienced any problem with the ring tone. If your phone shows a security notification when you are installing the file, please ignore it and continue with the installation."
As you've just recently bought your phone, this patch may already be installed. However, you could always give it a try!
You can download it from here, its the first option from the list:
http://europe.nokia.com/A4144780
You'll need your data cable and Nokia PC Suite installed.
Hope this helps!