can you try reseat the SIM card and Make sure your phone is off before removing your SIM card,
1. power off your device
2. take out the SIM card,
3. waiting a 1 minute
4. put it back and power on,
5. toggling airplane mode off/on,
do Rebooting your phone ,This is quick and simple,to refreshing your network,
*what is mean for Rebooting the phone means to turn off your phone and turn it back on again.
To access voicemail messages from your mobile device, hold down the ‘1’ key on your phone keypad. You can also access your voicemail from a landline or any other phone line, by dialing your 10-digit phone number, selecting ‘Send’ or ‘Call’ and then pressing ‘*’ to skip the system greeting.
Make sure you press "*" once your call to voicemail connects. You will then be prompted to enter your 4 digit voicemail pin #.
There is an option to NOT have a PIN number when answering using the device with SIM card. Why bother when you have a pass code on the device?
Has the “call voicemail” prompt stopped working?
calling my own number and giving a password does work.
It should still be working. I suggest you perform a Reset of your Network Settings on your device.
To Reset Network Settings, begin by opening the Settings app. Next, tap General -> Transfer or Reset iPhone -> Reset -> Reset Network Settings. and enter your passcode.
Your iPhone will reset network settings and restart itself.
Find and tap Settings > System > Advanced > Reset options > Reset network settings.
This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
You can also try to delete your voicemail number from your contacts, then re-add it as a new one. See here for Voicemail Access Numbers: