We've seen into in film and tv: The heroes or villains (when portrayed as the government) get your phone number and connect a call, then track you via that connection. In real life, my phone number has been mine for like...11 years. My work numbers have changed based on changes in...err...work, but if you know my # you can get a hold of me. Thankfully as a cell phone its not immediately available in a phone book, though like everyone else, whenever we have to use our number on some form, it increases the chance of spam as it gets onto other lists and other lists and other lists.
So then we look to some of the (at this time, only SR4 and before adventures, since there aren't any SR5 ones yet) adventure modules and see where various contacts and the like are calling the runner seemingly directly. It occurs to me how long term potentially dangerous that is for a runner, so how do they otherwise manage so that doesn't get them killed? Granted, as a game mechanic, it doesn't really matter, the GM can just go "Yeah they get in touch with you" and basically gloss over the Fake ID stuff as you flip through them as needed, but it made me think of the underlying narrative around it.
"Burn number"? Also in tv and film we have the 'burner phone' where no one has the number to the big bad's phone and he uses it to keep connected through collateral contacts, or the hero spy has one as a clean-phone. When compromised (too many people have the number, the owner knows the number will show up on the guy they just made dead phone, Big Brother does a cold call, whatever) they ditch it and find another.
Possible use for a Runner, especially in context with attaching it to a fake ID, so you have your fake ID and a commlink set up and then ditch it if burnt. The downside being that you'd want to make it disposable, and if disposable it can potentially be difficult for important people like a Johnson or your Fixer to get in touch with you until they know the new number. Likewise, a former contact (and typical GM story angle) won't be able to use that same 'number' to get in touch with you.
"Message service"? You have a number stored in a host somewhere that can take incoming calls and then that gets routed to whatever actual commlink/code you're currently using to receive. Acting like a matrix switchboard it connects you to the number but doesn't directly compromise you unless whoever is looking for you has wormed/brute forced their way into the host location to see what numbers this contact number is connected to. The recipient can know who is calling, but the caller only knows the 'public' number. In this case the protection depends on how good a particular host is. It also allows for multiple regions. You could have a public (sorta) number for your contacts in a particular region of the world and have multiple ones for different regions so you don't necessarily lose your entire web of contacts if one is compromised.
Any other thoughts?