Your Call World and iDial365
 

I may buy a T-Mobile PAYG SIM when in the UK next week. Can anyone comment on these services?

Do you dial the leading 0 in the access numbers as in (07755 229 365)

Yes, all ordinary mobile and landline numbers in the UK start with 0

Or you can save/dial all the numbers with a plus sign and country code (i.e., +447755229365). If you use the SIM in more than one country, you should always do this so that the numbers dial correctly.