The adapter often seems to be missing from the VM kit of parts as-supplied from some past posts on here.
The forum team has sometimes arranged for missing adapters to be dropped off by a local technician. However, it is a cheap part so you may want to just get one yourself more quickly. The adapter is shown in the bottom left pic. here
You need a RJ11 plug (which goes into the hub) to a BT socket (where you plug in your phone). Do a Google search and buy from your preferred online supplier. They should cost just a few pounds. They are common, generic parts so those which cost more are not necessarily of any better quality.
Some types of phone (mostly old equipment) require the adapter to have a ringing capacitor inside to make the phone ring (which would be in the adapter product description). Most modern phones though do not require this and a standard adapter would do.
If you want to wait for the forum team to respond, they might be able to arrange one for you. They can take upto a week to reply.