A friend told me that:

The exact approach used to break the encryption of Sky ECC phones is not fully detailed in the sources I found. However, there are some insights into the methods used:

1. One source mentions that law enforcement agencies used cloned devices running a fake phishing application designed to impersonate the Sky ECC app https://www.bleepingcomputer.com/news/security/europol-unloc.... This allowed them to intercept messages as they were being sent and received.

2. Another report indicates that unauthorized devices with modified security features were sold through unauthorized channels, which likely played a role in the interception https://www.vice.com/en/article/sky-ecc-decrypted-hacked-pol....

These methods suggest that the encryption itself wasn't directly broken, but rather the security of the devices and the integrity of the app were compromised.