I was using: https://www.localphone.com and when you sign up, you get one
international number for free, that looks like: +88351000XXXXXXX and you can
add many international numbers. But I am not sure if that is for me, as they
use non-free Javascript on the website, even though it works with LibreJS
You can connect over SIP to other users.
Best solution is to host your SIP server yourself, and acquire a number from
someone, if you wish to buy, like from Twilio for example.
Even better solution would be to simple host it yourself in your office.