@Deb66: Voicemail is a mobile network service therefore I recommend contacting your network directly to enquire about setting it up. In the phone itself your can go to Phone > Tap the 3 dots in the top right > Settings > Voicemail settings if you should need to enter the voicemail number manually. Hope this helps.
901 may be your voicemail number, @Deb66. Try calling that number and if that doesn't work then give your network a call to ask them about the necessary steps to access your account.