Can’t access Voicemail

A couple of months ago I got an iPhone 13 and signed up for a new service provider.  Ever since I haven’t been able to get Voicemail.  If I dial Voicemail I just get a short series of beeps. Same thing if anyone tries to ring and leave a message.  It is possible to ring the provider and turn on Voicemail but I just can’t access it after that.  Have tried all the advice on line (i.e. turn phone on/off; turn wireless mode on/off), none worked.  Service provider say everything is okay their end and it must be a problem with the phone.  Anyone any ideas?