The only testing that the operators are interested in (i.e. though people who will be actually selling it), is ..
Which actual phones has it been tested on.
They are not interested in knowing if it will probably work on device X.
There are places that will do this sort of testing, but they are expensive.
Better off buying handsets on eBay.
The following statistics are somewhat out of date, but should give you a rough idea of what phones to go for:
These are the statistics for 2004 from a reliable source.
Code:
Dec: Nov: Oct: Sep: Phone:
17.13% 18.08% 18.31% 20.25% Nokia Series 40
12.13% 10.93% 11.53% 11.26% Motorola V525/V600/…
11.40% 11.86% 8.95% 9.72% Nokia Series 40 (2)
7.60% 7.99% 9.08% 9.94% Nokia 6600
7.39% 7.92% 7.80% 6.45% Samsung E700
5.52% 5.48% 6.33% 8.29% SonyEricsson T610/T630
5.40% 4.88% 6.11% 5.81% Nokia Series 30
4.43% 5.98% 5.34% 2.12% Sony Ericssson K700
4.30% 2.69% 3.32% 2.45% Sagem MY-V65
3.38% offline offline offline Siemens CX65
2.73% 1.47% 1.65% 1.75% Nokia N-Gage
1.72% 2.50% 2.57% 3.15% Nokia 3650
1.70% 2.03% 1.94% 2.48% Siemens S55
1.67% 2.19% 1.99% 3.05% Nokia 7650
1.44% 1.70% 1.95% 1.97% Siemens MC60
1.35% 1.78% 1.89% 2.29% Samsung P400/E710
1.30% 1.06% 1.18% 1.57% Sharp GX10
1.22% 1.22% 1.21% 0.98% Nokia 3410
1.16% 0.97% 1.21% 1.41% Sharp GX20
1.13% 0.81% 1.12% 0.64% Sharp GX30
0.89% 0.56% 0.68% 0.59% SonyEricsson P900
0.73% 0.54% 0.68% 0.71% Siemens SX1
0.61% 0.74% 1.04% 1.04% Siemens M55
0.43% 0.60% 0.52% 0.45% SonyEricsson Z600
0.28% 0.21% offline offline Samsung X600
0.19% 0.23% 0.22% 0.22% Siemens M50
0.19% 0.14% 0.20% 0.31% Siemens C55
0.10% 0.08% 0.20% 0.22% Samsung X100
offline 3.17% offline offline Siemens SL65]