Well, not all phones are equal and not all TVs work the same. There's a difference in the reading of a TV versus a headset. Ideally it should work every time.

For example, some TVs have power saving features so they only detect at startup or "every now and then". As a result, it can be intermittent. My old CRT gets detected every time because back then "large TV" and "economy" went together like "truck" and "surgery".

New plasma though gets a bit irregular, so what I do is start the plasma with the N900 attached. Works every time. There is no indication on the Nokia that it detected it but it works, and sometimes there's a hint sometimes there's not.

Additionally, it's surprising how often people misconnect this stuff, because Audio Left, Audio Right, Luma, Delta Red, Delta Blue, Composite and outputs all have RCA. To make it confusing other standards also use RCA, To make stuff even more confusing, some TVs have no color codes. Others have "smart" jacks that try to guess signal. On the other side, another "smart" device tries to guess too.

N900 has a Composite out (yellow), with Audio Left and Audio Right (red and white). Most TVs have YPrPb connectors in the back, some only have that, and rely on SCART connectors to use "intelligent" switching between RGB, YPrPb and Component. An alarming trend with the new TVs is they all have unlabeled "inputs" that go automatic.

Perhaps one should invest in a Composite to SCART connector (it's passive and some 5$) to get reliability. Make sure you get the colors right. Oh, and, SCART is used for input and output, too, so make sure you read the direction right.

Basically,

a) If you get intermittent detection, you might have a smart system. Powering the TV with N900 plugged in could fix it. Also, a good adapter that is "for Composite" might hardwire the detection pins.

b) Image is black and white, maybe a bit blocky or shakey. You used a Luma port. The connector is a Component port, yours is Composite. Radically different, but if you use yellow on green you get a B&W (bad) image. All other combinations fail. Use a Composite port.

c) You have an adapter and it doesn't work. Your adapter might be RGB (need a new one if it's intentional), it might be Delta Coded (YPrPb AKA Component, yellow on greed is black and white, you need another adapter), it might be an output instead of an input (if it has a direction switch, use it, otherwise, new).