It seems that the cellular service providers (SPs) have decided that they are not making enough profit on Pay-AS-You-Go operations so over the last few months have made changes to charges and to how infrequent usage is calculated. This is largely down to the fact that many PAYG users don't use their phone much or indeed never use it so their number could be reused to help alleviate the number shortage across the board.
As far as I can find these are the 'rules' of the four main SPs, Vodafone, O2, EE and 3:
With Vodafone, EE and 3 you must have one chargeable event on your account at least every six months, that is make a connected call, send a text, or use some data.
O2 is the same except that the only acceptable chargeable event is a connected telephone call - sending a text or using data is no longer sufficient. This does mean that if you use such a SIM in a mi-fi unit you will need to have a phone into which you can insert the SIM at least every six months so you can make that vital call. If there is a size problem (mi-fi units usually take what these days we call a large SIM: if so you can buy small adapter plates into which you can insert your smaller SIM to make it up to full size for use in the mi-fi.
As for charges, this is a minefield:
One of the best PAYG deals was 3-2-1 on the 3 network, that is 3p/min calls. 2p/text, and 1p/MB of data. They announced last year that they were ceasing the sale of new 3-2-1 SIMs but (IIUIC) anyone who buys a new 3-2-1 SIM retail (particularly a pre-loaded SIM) and registers it before the end of this year will be able to use it as 3-2-1 until it expires. Other users on and remaining on a 3-2-1 SIM will see the prices rise to 10-10-5 from 16th February.
EE and O2 are unchanged. If you have a O2 Classic SIM which was also 3-2-1 that will continue to work as 3-2-1 as long as you keep it going even though they pulled the plug on new users not long after 3.
The big change is Vodafone. Until January 12th this year they charged 20p per event, that is per minute, per text, per MMS, or per Mb of data up to a daily limit of £1 and thereafter unlimited. As of January 12th they will charge £1 for every day that you use your phone between 00:00 and 23:59, starting from your first usage of any type and thereafter unlimited calls and texts and 50MB of data until 23:59. You only get charged on days on which you effect a chargeable event, receiving calls or texts doesn't count. Users need to be aware that if a phone is on it may use data in the background such as app or social media updates which would trigger charges. Best answer to that is to turn mobile data off until you can use wi-fi.
Anyone who may wish to change to a different PAYG SP should look at Pete Forman's excellent pages at http://payg.pythonanywhere.com If you should choose to move elsewhere do make sure what the usage requirements are. For instance there used to be a SP called, I think, Delight Mobile who were on EE and operated as 3-2-1 that required you to top up by at least £10 every 80 days, and if you failed to do so your number and credit disappeared immediately. The only 'keep it going' action was the top-up - usage was not enough. GiffGaff (owned by O2) is consistently regarded as the best PAYG bundle SP although their 'out-of-bundle' charges are relatively high.
As far as I can find these are the 'rules' of the four main SPs, Vodafone, O2, EE and 3:
With Vodafone, EE and 3 you must have one chargeable event on your account at least every six months, that is make a connected call, send a text, or use some data.
O2 is the same except that the only acceptable chargeable event is a connected telephone call - sending a text or using data is no longer sufficient. This does mean that if you use such a SIM in a mi-fi unit you will need to have a phone into which you can insert the SIM at least every six months so you can make that vital call. If there is a size problem (mi-fi units usually take what these days we call a large SIM: if so you can buy small adapter plates into which you can insert your smaller SIM to make it up to full size for use in the mi-fi.
As for charges, this is a minefield:
One of the best PAYG deals was 3-2-1 on the 3 network, that is 3p/min calls. 2p/text, and 1p/MB of data. They announced last year that they were ceasing the sale of new 3-2-1 SIMs but (IIUIC) anyone who buys a new 3-2-1 SIM retail (particularly a pre-loaded SIM) and registers it before the end of this year will be able to use it as 3-2-1 until it expires. Other users on and remaining on a 3-2-1 SIM will see the prices rise to 10-10-5 from 16th February.
EE and O2 are unchanged. If you have a O2 Classic SIM which was also 3-2-1 that will continue to work as 3-2-1 as long as you keep it going even though they pulled the plug on new users not long after 3.
The big change is Vodafone. Until January 12th this year they charged 20p per event, that is per minute, per text, per MMS, or per Mb of data up to a daily limit of £1 and thereafter unlimited. As of January 12th they will charge £1 for every day that you use your phone between 00:00 and 23:59, starting from your first usage of any type and thereafter unlimited calls and texts and 50MB of data until 23:59. You only get charged on days on which you effect a chargeable event, receiving calls or texts doesn't count. Users need to be aware that if a phone is on it may use data in the background such as app or social media updates which would trigger charges. Best answer to that is to turn mobile data off until you can use wi-fi.
Anyone who may wish to change to a different PAYG SP should look at Pete Forman's excellent pages at http://payg.pythonanywhere.com If you should choose to move elsewhere do make sure what the usage requirements are. For instance there used to be a SP called, I think, Delight Mobile who were on EE and operated as 3-2-1 that required you to top up by at least £10 every 80 days, and if you failed to do so your number and credit disappeared immediately. The only 'keep it going' action was the top-up - usage was not enough. GiffGaff (owned by O2) is consistently regarded as the best PAYG bundle SP although their 'out-of-bundle' charges are relatively high.