You can make the same sort of call but replace tel://
with telprompt://
so it will look like:
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:[NSString stringWithFormat:@"telprompt://%@", self.phoneNumber]]];
This will return you to the app once you have finished your phone call. This will work across most if not all iOS versions.
EDIT - 14/07/2015
I have been informed in the comments below that using telprompt://
is actually unsupported by Apple so they could decide to change this anytime and it could affect your app. So my suggest would be to wrap this call in a canOpenURL:
and see if you can open the URL before hand and handle it if you can't.
I'd probably actually go with checking this at App load up and setting it to a global variable that you can check when constructing your UI and maybe just not offer that functionality at all if you can't do it which would create a better user experience.
Big thanks to NicolasMiari
for pointing out the error and supplying the link to iOS: return to app when the call fails