VoxSyn is described as 'vocal effect tool for iOS with a 22-channel Vocoder, Audiobus and Inter-App Audio support for easy use with other audio apps' and is an app in the Audio & Music category. There are four alternatives to VoxSyn for Windows, Mac and FL Studio. The best alternative is Vocodex. It's not free, so if you're looking for a free alternative, you could try TAL-Vocoder or mda Talkbox.
TAL-Vocoder is a VST/AU plugin that emulates the sound of vintage vocoders from the early 80s. It provides 11 bands and users analog modeled components in combination with digital algorithms such as the SFFT (Short-Time Fast Fourier Transform).