Porting-voicemail problem

I recently switched from Sky Mobile to Smarty and ported my existing number across.
During the porting process, Smarty provided a new SIM. For about two days the SIM was active with its temporary number, and then my original Sky number completed the port and started working on the Smarty SIM.
In that two-day window, four voicemails were left on my Sky voicemail box. I could not access them because I'd already moved to the Smarty SIM, and now that the number has ported those messages aren't accessible via Smarty voicemail.
However, my iPhone Phone app is showing a persistent voicemail notification/badge (4 voicemails) that I can't clear.
I believe the message waiting indicator (MWI) is stuck from the port.
How do I reset/clear the voicemail notification / message waiting indicator (MWI) on my line so the iPhone stops showing 4 voicemails?

Date the port completed: 16/12/25
iPhone model / iOS version iphone 13 Pro, los 26.2:
I tried the following: (as advised by customer service)
In notifications - turned off the Badge icon
Got a friend to send me a voicemail, then deleted that message.
Went back to notifcations turned the badge back on, then restarted the phone.
But that made no difference