Voicemail/visual voicemail

If you forget your voicemail password, there's no way to reset it from your iPhone. You need to contact your carrier to reset your password. If you insert a new SIM card, you might lose the voicemails that you saved on your iPhone. Set up Visual Voicemail on your iPhone - Apple Support

Voice Mail (Visual or Normal) is a Carrier Specific feature. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.