You will need to pay VAT if:
Your company is registered in the United Kingdom, or
Your company is registered anywhere outside of the EEA and you have users in the UK
When only some of your company’s users are based in the UK, VAT applies in proportion to the amount of use from the UK.
Your UK company is using Taxi and your users are mostly in the UK. Sometimes your team use Taxi while travelling abroad on business trips. VAT is charged at 20% because use outside of the UK is negligible.
Your American company is using Taxi in the USA. No VAT applies.
Your company is registered in France. Your marketing team are all in the UK. No VAT applies because you are not outside the EEA.
Your company is registered in the British Virgin Islands and your marketing team use Taxi from your office in London. VAT is charged at 20% because your Taxi users are in the UK.
Further explanation of VAT rules as they apply to Taxi for Email are explained in section 13 of VAT notice 741A, which you can read on the GOV.UK website.