need help with adding sim questions

good day folks,I have unlimted wifi in my wifi dongle to my pc (£20) no problems there.I would like to add a unlimted calls and texts (iv just lost my job on the 19th of feb) to my mobile phone (ie i have a sim here @ £10-40gb data,unlimted calls/texts (i want to keep my wifi dongle separate to my fone as my sony experia 10 iii does not accept the 4g lte,beleive me iv tried),so i have the pac code from o2 (but ill do that later on here) how can i add this new mobile fone number to my account? 1,just pay one account on here,2,change the £10 to the £6 unlimted calls/txts?,these sims are for my usage only,my o2 is trash,i cant get a signal barely,tanks for your help,jim