# Contact Sync (Legacy) ## Introduction Contact Sync is an advanced, optional feature of Cloud Softphone which uploads contacts from a mobile device to Contacts server and keeps the server-side copy up-to-date. The server-side copy of contacts is used for Smart Contacts. The server checks which people from the address book use the same service and offers on-net calls with extra features like video calls or file transfers for such contacts. ## Requirements ### Contacts server The server which stores the contacts and provides APIs to clients to keep the contacts in sync. The server is available either as a binary for Linux, installable on premises of provider, or as a hosted solution by Acrobits. For providers who prefer hosting their own Contact Sync server, see the [installation instructions](https://doc.acrobits.net/server-components/contact-sync/). ### SSO server Single Sign On server, which validates SIP `username&password` using [ext_auth](../provisioning/ext_auth.md#ext_auth) web service and creates access tokens, which clients use to authenticate themselves. This server is also available either as a binary for Linux to be installed on premises of provider, or hosted by Acrobits.