This is still happening when I phone from indoors so have had to change my landline so its free every day,
I notice that when I am indoors the 3g mast im using is about 5 miles away but I only have to walk 50 yards or so and I pick up a local mast
I have bought a pay as you go Lycamobile sim , which uses o2, and the 3g signal is about 20db stronger, using a local mast, and I have had no problems.
I understand from other postings that 3g is required for speech
Looks like I will have to try to cancel the Virgin contract