Translation:That is my phone number.
Why the 个 here? I've seen other sentences with just 那是 without including the counter.
They're different mas; 吗 ma (fifth/neutral tone) is the question marker; 码 mǎ (third tone) is part of the word for number, 号码.
Isn't the 个 just used to refer to people? In this sentence, I think the 个 is refering to the phone number, not the subject 我.
个 refers both to people and it's also the generic measure word for when there isn't a more specific one, or you don't know it. (It's better to be more specific if you can, but I think most Chinese speakers understand measure words are tricky for learners.)