SIP is the abbreviated form of session initiation protocol. It is a telecom protocol which is often applied to Internet-based telephony. The SIP controls the placement, maintenance and closure of a call.
Many Swiss telecom service providers give their customers access to their SIP. This allows customers to make calls from their number and receive calls to their number from anywhere in the world, as long as they have Internet access. This allows them to continue using their phone service while traveling.
There are SIP apps which can be installed on smartphones. These apps allow you to make calls from your regular Swiss landline number on your mobile phone.
Not all Swiss telecom service providers allow their customers to access their SIP. Customers of these service providers can normally only make calls through their SIPs from their homes.
The moneyland.ch landline plan comparison clearly shows which plans provide SIP access.