It's a Virgin Media issue. This has been driving me nuts for two months. User R101 on the Virgin forum posted this yesterday and it works!
https://community.virginmedia.com/t5/Virgin-Mobile/iPhone-Dial-Assist-Problems/td-p/3795425/page/7
I've found a way to replicate this issue at will and am currently experimenting with a fix.
The potential fix I'm experimenting with is to do the following.
Go to:
Settings -> mobile data -> SIM Applications (near bottom) -> tap "Roaming" -> tap "Select Mode"
I found "automatic" was selected by default.
Tap manual -> then choose National.
Now make sure your numbers are set to the correct format. If not change region to anything other than UK (Settings -> General -> Language and Region -> Region) and then change back to UK region.