You may also wish to check your contract T&C's http://www.o2.co.uk/termsandconditions
2.2 The Service isn't fault-free; a range of different geographic, atmospheric or other conditions or circumstances beyond our control can impair it. For instance, coverage is affected by things like the thickness or material of the walls of the building you're in. It might also depend on how many people near you are trying to use the Service at the same time. For more information about the things that affect coverage and data speeds, please look at the dedicated Network pages of our Website at o2.co.uk/network. You're entitled to the quality of service generally given by a competent mobile telecommunications service provider, using its reasonable skill and care. We will attempt to re-perform disrupted Services when possible. We may direct and manage traffic on our Network as is required to deliver this quality of service, including in some circumstances directing traffic onto different components of our Network such as 3G, 4G, 5G and wifi. You can disable wifi in the settings on your Mobile Phone or possibly other Equipment if you do not wish to use that part of our Network. We may also carry out upgrades, repairs and maintenance works to our Network from time to time. Sometimes technical issues, impaired quality of service or outages on our Network can occur. If something goes wrong, we'll try to fix it quickly. If you experience continuous or regularly recurring disruption to the Services we will endeavour to fix the Services and you may be entitled to a partial credit of your Monthly Subscription Charges to cover the period that your use of our Services was disrupted. To receive a partial credit, please report this disruption to our customer services team by calling 202 free of charge from your O2 mobile. We will assess the disruption against your typical usage history. We may offer you alternative products or services to address the disruption, and an alternative means of accessing the disrupted Services must not be available to you. If our Services are materially degraded for an unreasonable period of time you may be able to leave your Agreement with us early without paying an Early Termination Charge (although you’ll need to pay off any relevant Device Plan). Contact us to discuss your options.