Three is cheap for a reason
The network is good when it works. That's it as far as pluses go. Customer service is without doubt one of the worst I have ever experienced of any company. There seems to be little interest in customer retention and issue resolution. One hint for the latter would be to try and get to someone who is empowered to make decisions, as first line staff seem to have little ability to do this. Official complaints handling is average if somewhat lengthy. Their service is cheap to purchase and this is reflected in the quality of service offering. For services such as telephony my preference would be for reliable services with issues resolved within a reasonable amount of time. 3's processes are not effective enough to offer reliable service and solve issues quickly. Customer satisfaction would appear not to be a priority for this company. None of my interactions with them has been without fault. If dealing with 3, check T&Cs carefully and be prepared to escalate if they do not stick to them - or go elsewhere.