We just published an integration between Frappe/ERPNext and Sipgate, a German provider for Voice over IP telephony. The integration can sync contacts from Frappe/ERPNext to Sipgate.
You can configure which types of contacts you want to sync. For example, you can decide to sync only contacts belonging to your customers and suppliers.
Check out the ERPNext integration with Sipgate on Github. Let me know if this is useful to you and feel free to contribute!