When I spoke to T-Mobile recently they said I couldn't use a PAYG sim for Pay Monthly. I have a Three PAYG sim, would I be in the same position for Pay Monthly (having to wait for another)?

What difference does this make?

I fail to see why this would bother anyone?

Because I'll not bother setting it up, noting the number, etc if I've then got to order another with a different number and wait.

Not sure why the rudeness

I wasn't being rude. I was asking why it'd bother you.

It's the same for all networks i think.

Don't think the OP was being unreasonable in asking the question. It seems logical that they could just switch a PAYG sim to pay monthly tariff (assuming the sim card was compatible).