Huh?

So... I just had my first entirely helpless foreigner moment. I'm sitting on the train on my way into HK, and my phone rings (my local mobile phone). It's an unknown number, but a few numbers here, as well as calls I receive from America, are unknown. So I answered it, and almost immediately a polite sounding man started speaking to me in Cantonese. Being the only westerner on the train, I was somewhat self conscious about speaking Cantonese (I'm usually not, but was feeling incompetent anyway) and after his little 30 second spiel I asked him to speak in Mandarin. So he started over and I still didn't catch a whole lot (it's hard to understand people over the phone to begin with, especially on a crowded train), but was almost positive I caught words and phrases like "our company" and "credit card" and "cheaper" and "convenient." I'm used to recognizing the de rigueur egregiously fake friendly quality in an American telemarketer's voice. However, a friendly voice in a foreign tongue (especially Cantonese) doesn't yet carry any ulterior motive.
I was almost positive he was a telemarketer and repeatedly said "no thank you no thank you" (不要，不要 and 沒有興趣） and he seemed genuinely sorry to bother me and said so. I couldn't bring myself to hang up on him, but felt terribly rude all the same. He didn't address me by name like any of the friends here would, not even any of the brothers or sisters who I need to speak to and as of yet have not met.
But I got to thinking: What if he's the brother at the British Council I've been wanting to talk to, or someone at the bank I need to speak with, although I was sure it was a solicitation. I was confused and worried, and pulled out my ipod in lieu of continuing to read my book.

PS: I got another one on the way home. It was also an unidentified caller, and I was more confident this time. I told him I didn't speak Cantonese and that he'd have to speak in Mandarin or English. He said 不好意思 and thankfully hung up.
I don't know...