This subject was raised by you before in a previous post
so hopefully Monzo will give some thought to PhatPheddy’s suggestion of adding a UK landline instead of trying to call +44800
Alternatively you could have a 00800 number instead of an 0800 number (an International Freefone number instead of a UK Freefone numver) or have a UK 03xx code number.
Both +800 and +443xx are connectable from all countries in theory, but I would opt for +443xx as a safer bet as some countries seemed slow in connecting to international Freefone +800 but all will connect to +443xx and when dialed from within the UK 03xx is often included in call allowances on certain networks.