If you can't tell what they're saying, I wouldn't worry about responding specifically. Probably best to just say sorry the connection's not great but they should check out the website, maybe there's some line from the script that sounds good, improvise, or some relevant sound-byte from :citations-needed: or something.

I phonebanked for Bernie back in 2015, if it's similar nowadays most people won't answer, so you'll get desensitized to the ringing. Even though it's electoralism, I'm sure whatever you're phonebanking for would materially improve anyone's life or at least not make it worse like the alternative, so you're just trying to help them, and it's just a minute or two so anybody who gets mad about getting a call on a Saturday can fuck off.