O2 has joined EE and Vodafone in offering the cellular version of the Apple Watch Series 4.

New and existing iPhone pay monthly users can now purchase the smartwatch through O2 on a number of price plans, and use it to make calls and stay connected, even when not in range of their handset.

When Apple first launched an LTE version of its Watch (with Series 3) it exclusively partnered with EE. Vodafone connectivity was added at the end of last year. Now O2 supports the device.
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https://www.pocket-lint.com/smartwatches/news/o2/148359-o2-customers-get-apple-watch-series-4-at-last