Siri calls random numbers even though it is deactivated
If you disable Siri iOS still leaves VoiceControl enabled. This is the basic pre-Siri voice-control system on the phone for queuing music and dialing by voice without needing an internet connection.

There is no publicly documented way to disable it completely short of disabling cellular access (i.e. airplane mode).

However, you can stop calling from the lock screen: 


Open Settings
Open General
Open Passcode Lock (which is named Touch ID &amp; Passcode on newer devices)
Turn off Voice Dial


Now, if you press it, it will tell you calling is disabled. Not perfect, but better than calling random numbers.

Alternatively, if you just want to stop Siri pocket dialing, you can turn Siri back on, and then on the Passcode Lock screen,you can disable access to Siri from the lock screen. That prevents Siri from appearing when locked completely. Might be a better solution if the pocket dialing is your underlying issue.