The iPhones use a different operating system, so they behave differently to changes in wireless networks.
Suggest that you trash the current Wi-Fi settings on both iPhones and then set them up again, as I helped a neighbor with the same issue and it was resolved by this procedure.
Of course, there are no guarantees that it will work for you, but it might be worth a try if you have not already done so.
On each iPhone.....
Touch the arrow to the right of the name of your network
Touch Forget this Network
Power off the iPhone
Move the iPhone close to the Time Capsule
Power up the iPhone and set up a new connection
Check for proper operation
It sounds to me like you may have a DNS problem. After doing as Bob suggested and forgetting the networks on the iPhones. When you setup the new connection
Touch the arrow to the right of the name of your network.
Look at the IP Address information, DHCP should be highlighted.
Look at the IP address and make sure it is in the range assigned by your network router.
Look at the subnet mask it should most likely be 255.255.255.0
Look at the router IP address, it should be the same as the one that your airport utility says you assigned to your Time Capsule.
Look at your DNS IP address, it should match the DNS address(es) of your working iPad, MacBooks, etc.
Look at the search domain, it too should match a similar setting in a working device.
Finally, in most cases your HTTP Proxy indicator should be OFF
If any of these settings are not as stated above, change them and report back with the results.