Also, keep in mind to check you E164 rules:
3cx Admin -> Settings -> E164 Processing
Make sure to select:
United Kingdom: +44
check - remove if same country
International Dial Code: 00
National Dial Code: 0
those settings did it for me, thus outbound rules considered irrelevant in this scenario for formatting the number.
Furthermore, a +44 rule would show up as 01144 when hitting the outbound rules.
If you require any help, message me.