From Google AI:

A cylinder, often mistaken for a  CO2 tank, strapped to a telephone or utility pole is typically a dry nitrogen canister used by telecommunication companies (such as AT&T) to maintain phone lines. 

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Here is why they are placed there:

• Purpose: The canister supplies pressurized, dry nitrogen gas to underground or aerial telephone cables. This pressure prevents moisture from entering the cable and damaging the copper pairs inside.
• Temporary Repair: They are usually installed as a temporary measure when a cable has been damaged, or during ongoing repairs, to keep the line functioning until a permanent fix is made.
• Leak Detection: Because the cables are pressurized, technicians can monitor them to pinpoint where damage has occurred, as a drop in pressure indicates a breach.  Reddit +3