Been waiting on this one for about a year, and have been asking for it. A function to skip steps within sequences when there is missing data, or to move contacts within a sequence would be extremely useful. As it stands, one marketing campaign must have at least two sequences if you have some prospects with phones, and some without, if you want it to be fully automated.
Currently, since I like to differentiate people with work emails from those without, this means for one marketing campaign I need 4 sequences for it to be entirely automated. This skews stats and in general seems like an unnecessary hoop that should have been fixed long ago.