For me O2 for this reason....
Used to be with Vodofone. topped up with tenner a month, quickly ran out of credit, never lasted me the month.
Now,
Top up mine and wife's phone with 10 pounds each month.
300 free text's....keep my tenner for calls/whatever
15 mins included talktime to landline/O2 for being with them 6 months(treats)
Always build up a balance on my phone, free to make phone calls now without worrying I'll run out of credit. Home phone bill is now *very* low as we NEVER use it for mobiles now.
Altogther we get more freedom with our mobiles and overal including landline, we are cheaper/month.
Rarely, if ever get problems making calls, sending/receiving texts, etc
Hope ths makes sense !