So after interaction with Steven_L over a few days and trying lots of options on both mine and partner's phones, the issue is now resolved. The problem seems to be that after my partner enabled WiFi calling on his phone, I could no longer send text. This was resolved by me deleting the contact entirely on my phone and then setting up the contact again. Originally the contact had 3 different contact numbers listed, with mobile as the default and work/other also listed. Apparently this may be the cause of the problem. So implies contracts with multiple numbers don't work when the recipient changes to WiFi calling. Odd as phone contacts system still allows multiple numbers to be stored within each contact.