I've been here a while now, so things may have changed. But the concern, as someone mentioned, maybe that you'll do a bunk before paying ya bill. But it was possible for me (with SwissCom) with an "L", I just had to pay a bigger deposit than normal is all.
(I got a landline and mobile tho' and only paid 1 deposit for the two).
And it is a deposit...so you do get it back, after a few bills of you proving you cough up on time.