I think what your accountant is trying to replicate is this...
https://www.gov.uk/government/publications/vat-telecommunication-services-used-outside-the-eu/vat-te...
In 2017, UK VAT was added to roaming charges for consumers (but not businesses). This was due to a change in the interpretation of the Place of Supply for B2C.
I haven't read the full text but it looks like Place of Supply for B2B remained unchanged - which leaves it open to confusion. It would be considerably easier if phone companies charged B2C & B2B with UK VAT which businesses would then reclaim (I know our supplier - O2 - zero rates all roaming on our business account).
If your accountant insists it should be Reverse Charged then the code to use would be 20.0%RCSG (we are no longer in the EU so should not be using an EC/EU VAT Code ... & certainly not ECS0% which was for services that would be zero-rated anyway).
NB: the description in the above-linked thread VAT codes in QuickBooks Online: A complete guide is wrong - 20%RCSG is the Rest of World equivalent of 20.0% ECS - Intra EU B2B purchase of services.
Just to reassure slightly - whichever one of these codes you use, it will make no difference to the VAT you actually pay to HMRC:
Z=0% / Exempt both post Β£nil to to Box 4.
The RC codes post Β±VAT to Box 1 and 4 - which cancel out.
The codes obviously have different effects on Box 6/7 but these are less important to the VATman.
