Engaging with Communications: https://engagingwithcommunications.com/ – Lots of telecoms related content covering fixed and mobile communications, including mobile phones and network related papers.
GSM History: http://www.gsmhistory.com/ – Includes a fascinating history of the development of GSM from a standards and political perspective, along with coverage of the evolution of mobile phones and network standards.
Mobile Phone Museum: https://www.mobilephonemuseum.com/ – This is the go to site for information on mobile phones.
BT Archives: https://www.bt.com/about/bt/our-history/bt-archives
Science Museum Information Age Gallery: https://www.sciencemuseum.org.uk/see-and-do/information-age