Did that help at all?
You need to contact 48 support to get them to reset it as explained in the voicemail FAQ/guide:
Contact them here:
http://www.48months.ie/help#ask-a-48-agent
Make sure to choose a PIN that you will remember next time.
Hope this helps.
Did you read this tutorial?
What exactly are you trying to do (outline the exact steps that you follow in your phone) and what exactly goes wrong (verbatim error messages or screenshots etc.)?
Are you sure that you're not trying to configure soemthing like Visual Voicemail (which may not be supported by 48) rather 48's voicemail service?
https://support.apple.com/en-ie/HT201436
Hope this helps.
Edit: just Googled and "service unavailable" does sound like an iPhone Visual Voicemail error. I don't think that 48 (or any other Irish carrier?) supports that service but ask support to be sure:
http://www.48months.ie/help#ask-a-48-agent
Did that help at all?
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