Facetime Won't Connect - Dual Sim

I've recently added a second eSim to my iPhone 13 Pro. This is a data only sim on the Three UK network.

My primary sim (Vodafone UK) holds my number and is set as default for calls and sms. This is also the only number registered in iMessage & Facetime. I've double checked since setting up this new second eSim.

My secondary sim (Three UK eSim) is set as default for data.

Since adding this second sim, I'm unable to successfully make a Facetime call. I can however make and receive WhatsApp video calls just fine.

When I make a Facetime call out - my phone rings out as normal, however the recipient get's nothing.

When I receive a Facetime call, my phone rings, I accept and "join" the call - but just see the "connecting..." message for around 10 seconds before it times out.

Do I have to have the Vodafone sim set as default data for Facetime to work?? If not, does anyone have any idea how I can get it working?

Many thanks in advance!