Would consider using them but Taiwan is not a province of China (https://messagebird.com/en/numbers/). If a company has has many factual errors on their website like this, then I cannot trust their service to be reliable.
Let's make it more expensive for companies to give dictators what they want.
Taiwan has democracy, doesn't threaten its neighbours, has a working legal system, respects human rights, and has an educated population with access to the free world.
Calling it part of China is highly offensive and bowing down to the communist propaganda is simply unacceptable. They have Huawei as a client so choose money over morals.