Wonder if there is a specific pattern to add or remove as a prefix? Courtesy of Google Gemini below.
To make calls through Voipfone UK, you need to prepend 44 and remove the leading zero of the area code so that numbers are in the international E.164 format (e.g., dialing 02079460123 becomes 442079460123)
Thanks for the suggestion, but I don’t think so. If I set the same number up on a voip phone, I don’t need to add any prefix. In any case, the symptoms indicate earlier in the chain: i.e. something in FreePBX rather than at voipfone.