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There is always a volume control on any phone, either a push-button on the side (or two buttons close to each other, one for Volume Up or Volume +, the other for Volume Down or Volume - ... look for markings or labeling to that effect, perhaps in the cryptic terms of 'VOL-' and 'VOL+'.
If the Convoy phone model has no such buttons or no such markings, try pressing the VOL button that is the upper of the two buttons of that sort, which should increase the Volume.
And look in the controls on the phone - there should be a volume control app or function or setting, there.
On occasion - and it's not commonly encountered - the microphone in the phone is failing or almost disconnected (whether mechanically by separation of contacts, or through the contacts from the microphone being corroded esp. by damp or moisture getting into them, or other causes.) If so, this requires repair-servicing or else replacement of the whole phone.
(-Is the phone less than a year old? if so, warranty servicing would be free of charge.)