Yes, very widely used enterprise VoIP.

Edit: At least in North America, can’t speak for the rest of the world.

Have you literally never been in an office? They are probably the biggest manufacturer and supplier of telephony systems, at least in the UK. Well, since VoIP came along, anyway, before that it was all Nortel and BT Meridian, with the odd Avaya here and there.

The VoIP server software they use supports XMPP natively so your softphone on your PC is just a Jabber client, and (as far as I've been told) they use XMPP to sync between servers.

You've definitely used Cisco telephony kit, because you've definitely heard this tune: https://www.youtube.com/watch?v=pais41IW5dk

yes in fact they made iphones before apple. (apple stole the name and later licensed the trademark from cisco)