I do security testing at large scale clients and it is common for things like port and vulnerability scans against VOIP phones to cause these ghost calls. With your phone likely being directly internet facing, it is possibly being port scanned. These types of scans happen both automatically, but also can be the sign of someone targeting your device directly e.g. trying to find vulnerabilities on it or something similar.
Any device which is not behind a firewall should be considered a security risk.
VOIP Phones need to be purchased from a reputable vendor who provides ongoing firmware updates, these need to be applied ongoing.
Clients (large scale businesses) do not have this problem internally due to well defined firewall rulesets and they perform firmware upgrades by subscribing to vendor notifications.
I personally think Vodafone can do nothing and this idea of channels is nonsense.
Edited by ukhardy07 (Tue 19-Dec-17 00:49:27)