Have they changed their number?

[elmarsh]

If I get the “welcome to the o2 messaging service the person you have called is unable to take your call, please leave your message after the tone” 

Could that person have changed their number? Or would I get a different message, such as “number not recognised” 

[pgn]

If the number had been terminated, "number not recognised" is the default, or "number unobtainable", @elmarsh 

It is possible your called party has changed their phone and voicemail notifications are broken Guide: How to fix stuck voicemail notification 

[elmarsh]

Thank you. I wasn’t sure if there would be a delay and the voicemail still active for a certain period of time