Personal cross-device access is a different requirement from a business API.
The product in view
OpenBubbles describes direct communication with Apple and activation using one-time Mac access or an always-online iPhone. The site offers a free self-hosted option and a hosted service listed at $16.99 monthly, with a $1 seven-day trial. At the time reviewed, the page also said hosted devices were allocated.
Before you choose
Confirm current capacity before expecting to enroll in hosted service. Do not infer a business shared inbox, commercial messaging API or support commitment from a personal Android application. If the requirement is simply to read and answer your own messages on another device, that narrower use may be a better comparison. This directory does not present it as interchangeable with a managed business line.