Just to be clear, I am focusing on the UK here.
It should be noted that Tesco Mobile is an MVNO (Mobile Virtual Network Operator), so they don't have their own network but provide a branded service on top of a network operator, which in the UK is O2.
If you wish to change the number of rings, or more specifically amount of time before an incoming call diverts to voicemail then do the following:
**61*905*11*30# and then press call.
See Vodafone