Make SIP calls via API

I'd like the ability to initiate a SIP call from a device other than than the device running my SIP client.

For instance, via an API I could initiate a call from my desktop computer which automatically connected the call to a hardware desk phone or to a software client running on another device.

This API (with additional software) could allow a user to click on a tel:// URL or to select a number and launch a service to initiate the call. This would also allow for a "call me back" button on a Web site, for instance.

It appears that such functionality is available for Asterisk via the Asterisk manager API (http://www.voip-info.org/wiki/view/Asterisk+manager+API), but I think I'm correct in saying that AAISP are moving away from/no longer use Asterisk(?)