Yes, if you switch from Android to iOS or vice versa, you need to purchase the app again on the new platform. Groundwire and Acrobits Softphone require only a one-time purchase fee for download on the same platform.
You can install the app on multiple devices without repurchasing, as long as they use the same operating system and are linked to the same OS account.
There are no additional monthly subscription fees after installation.
For enhanced communication security, you may purchase an optional ZRTP plugin for the app. For more details, refer to the ZRTP Options section at faq.acrobits.net/srtp-on-acrobits-softphone-and-groundwire.