For mobile, when I switched to Galaxus Mobile (formerly Digitec Connect) they handled the switch and I could choose how it would go. I selected the option to make the switch at the end of my contract with Salt. They sent me the SIM card a bit before the switch and I had to activate it when I lost the Salt covering (I don't remember exactly, but I had less than a day of downtime). There was an option to carry on the phone number.