Thanks for that. I am running a google cloud server and got an error on apt-update (I'm not a linux guy):
...
Get:17
https://downloads.3cx.com Translation-en [1,245 B]
Get:18
http://deb.debian.org jessie/main Sources [7,063 kB]
Get:19
http://deb.debian.org jessie/main amd64 Packages [6,818 kB]
Get:20
http://deb.debian.org jessie/main Translation-en [4,581 kB]
Fetched 113 kB in 7s (14.1 kB/s)
Reading package lists... Done
W: GPG error:
http://packages.cloud.google.com cloud-sdk-jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
W: GPG error:
http://packages.cloud.google.com google-cloud-compute-jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error:
http://packages.cloud.google.com google-cloud-packages-archive-keyring-jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
W: Failed to fetch
http://packages.cloud.google.com/apt/dists/google-cloud-packages-archive-keyring-jessie/InRelease
W: Failed to fetch
https://downloads.3cx.com/downloads/3cxpbxexpress/Packages Hash Sum mismatch
W: Failed to fetch gzip:/var/lib/apt/lists/partial/downloads.3cx.com_downloads_3cxpbxexpress_en%5fUS.gz Invalid file format
W: Failed to fetch gzip:/var/lib/apt/lists/partial/downloads.3cx.com_downloads_3cxpbxexpress_en.gz Invalid file format
W: Some index files failed to download. They have been ignored, or old ones used instead.