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Signal Gone? Data Stopped? No Service? Get Solutions That Work
If your O2 phone suddenly stopped working, you have no signal, data is not connecting, or calls keep dropping, find step-by-step fixes below. Most problems can be solved without calling O2.
Understanding how mobile networks function can help you troubleshoot issues more effectively and have more productive conversations with O2 technical support. O2 operates a nationwide cellular network consisting of thousands of base stations (commonly called "masts" or "cell towers") that provide coverage across the UK. Your phone constantly communicates with the nearest base station to maintain your connection for calls, texts, and data.
When you turn on your phone, it searches for available networks and connects to O2 using information stored on your SIM card. The SIM card contains your unique subscriber information that identifies you to the network and determines which services you can access. Your phone then maintains a connection to the nearest O2 base station, automatically switching to different stations as you move around (a process called "handover"). This seamless switching is why you can make a call while travelling without the call dropping – most of the time.
Signal strength depends on several factors: your distance from the nearest base station, physical obstacles between you and the station (buildings, hills, trees), the number of other users connected to the same station, and interference from other electronic devices or networks. Indoor signal is often weaker because building materials (especially concrete, metal framing, and energy-efficient glass) can block or weaken radio signals. This is why you might have full signal outside but poor signal indoors, even in the same location.
O2 operates multiple network technologies simultaneously. 3G (third generation) provides basic mobile internet with speeds suitable for email and light browsing. 4G (fourth generation) offers significantly faster speeds, enabling video streaming, video calls, and more demanding applications. 5G (fifth generation) provides the fastest speeds and lowest latency, ideal for real-time applications and very data-intensive uses. Your phone will automatically connect to the best available technology based on coverage and network conditions. If you're experiencing slow data, your phone may have fallen back to 3G in an area with limited 4G coverage.
Network outages occur when base stations or network infrastructure experience problems. These can be caused by equipment failures, power outages, maintenance work, damage to infrastructure (such as cables being cut during construction), or extreme weather. O2 monitors their network continuously and typically resolves outages within hours. During an outage, you may experience no signal, calls not connecting, slow or no data, or text messages not sending. Checking O2's network status page or social media accounts can confirm whether an outage is affecting your area.
If your O2 signal has suddenly disappeared when it was working fine before, this is usually a temporary issue that can be fixed quickly. The most common causes are network glitches, phone software issues, or local mast problems. Follow these steps in order.
Go to Settings, turn Airplane Mode ON, wait 10 seconds, turn it OFF. This forces your phone to search for the O2 network again and often restores signal immediately.
Power off completely (not just sleep), wait 30 seconds, power back on. A full restart clears temporary network glitches that airplane mode toggle might miss.
Visit the O2 network status page or check @O2 on Twitter. If there is an outage in your area, O2 will show it. Outages typically last 1-4 hours.
Power off, remove your SIM card, check it for visible damage or dirt, gently clean the gold contacts, reinsert firmly, power on. A loose SIM is a common cause of sudden signal loss.
Put your O2 SIM in a friend's phone. If it works there, the problem is your phone. If it does not work there either, the SIM may be faulty or there is an account issue.
iPhone: Settings > General > Reset > Reset Network Settings. Android: Settings > General Management > Reset > Reset Network Settings. Note: This erases saved WiFi passwords.
iPhone: Settings > Mobile > Network Selection > turn off Automatic > select O2-UK. Android: Settings > Connections > Mobile Networks > Network Operators > select O2.
Nothing Working?
If none of these steps restore your signal, call O2 technical support on 0344 809 0202. They can check your account status, run remote diagnostics, and confirm if there is a local mast issue. Best times to call are 8am-10am or after 7pm for shortest waits.
If you can make calls and send texts but mobile data and 4G/5G internet is not working, this is usually a settings issue or you have run out of data allowance. The good news is this is almost always fixable without calling O2.
APN: mobile.o2.co.uk (or payandgo.o2.co.uk for PAYG)
Username: o2web (or payandgo for PAYG)
Password: password (or leave blank)
MMSC: http://mmsc.mms.o2.co.uk:8002
MMS Proxy: 82.132.254.1
MMS Port: 8080
If your O2 phone shows "No Service" or "Emergency Calls Only" at the top of the screen, your phone cannot connect to the O2 network. You can still call 999/112 in an emergency, but normal calls and texts will not work. This is often caused by account issues, SIM problems, or network outages.
Fix: Check My O2 app or call 0344 809 0202
Fix: Reinsert SIM or get replacement
Fix: Check status page or restart phone
Before troubleshooting further, verify your account is active:
If your O2 calls keep disconnecting mid-conversation, this is frustrating but usually fixable. Dropped calls are typically caused by weak signal, moving between cell towers, or phone issues. Here is how to fix it.
If calls consistently drop in specific locations, O2 needs to know. There may be a mast issue they can fix.
WiFi Calling lets you make calls over your home WiFi when mobile signal is weak. If it is not working, here is how to set it up and troubleshoot common problems.
If your O2 voicemail is not saving messages, callers say it goes straight to voicemail, or you cannot access messages, here is how to fix it.
901
Dial from O2 mobile (free)
1760
Deactivates voicemail
1750
Reactivates voicemail
Voicemail not saving messages:
Your inbox may be full. Dial 901, listen to and delete old messages. You can save up to 20 messages for 7 days.
Calls going straight to voicemail (you're not getting them):
Check: Do you have signal? Is Do Not Disturb off? Call O2 to check call forwarding settings on your account.
Cannot access voicemail - asking for PIN:
Default PIN is usually the last 4 digits of your O2 number. If forgotten, call O2 on 0344 809 0202 to reset it.
Voicemail not working abroad:
You can still access voicemail abroad but calls to 901 may be charged. Disable voicemail before travel by dialling 1760.
If your phone says "No SIM", "Invalid SIM", "SIM Not Detected", or "SIM Locked", here is what to do for each problem.
This usually means your phone is locked to another network and won't accept an O2 SIM.
This appears after entering the wrong PIN three times. You need your PUK code to unlock.
If your SIM is damaged or faulty, you can get a replacement:
Your phone may be set to 3G only mode. Go to Settings > Mobile > Network Mode and select 4G/LTE. Also check you're in a 4G coverage area using O2 coverage checker. Some older SIMs don't support 4G - order a free replacement if yours is more than 5 years old.
5G can sometimes be slower than 4G if you're at the edge of 5G coverage or the 5G mast is congested. Try switching to 4G mode to compare speeds. 5G coverage is still being rolled out and may not be reliable in all areas.
Delayed texts are usually caused by network congestion or phone issues. Restart your phone, check you have signal, and ensure your message centre number is correct (+447802000332). If texts are consistently delayed, call O2 to check account settings.
Phone updates can sometimes reset network settings or cause compatibility issues. After an update: restart your phone, reset network settings, re-enter APN settings if needed. If problems persist, there may be a bug - check for another update or contact O2.
Building materials like thick walls, metal structures, and energy-efficient windows can block mobile signal. Enable WiFi Calling to use your home WiFi for calls, move near windows, or consider a signal booster. Report persistent coverage issues to O2.
AnswersPro is an independent help resource and is not affiliated with O2 or Virgin Media O2. All information is provided in good faith to help O2 customers troubleshoot technical issues. For official support, contact O2 directly on 0344 809 0202.