You need a mature and responsible person to provide proper childcare. She isn't it.

OP, I'm very concerned about you given that you don't seem to think this is an issue. There's a greater debate about how much phone use is acceptable (with most MBs falling along the "none at all" to "occasional use or when kids are sleeping or occupied"), but here you have a nanny who WHILE YOU WERE HOME (so what does she do when you're gone???) left your young toddler in the basement for a long period of time so she could be upstairs having a phone call.

OP, no matter what you think about phone use, that is not ok. You are well within your rights to tell her this is not ok and that you expect her to be engaging your child and that phone calls and texting are to be kept to an absolute minimum unless your child is sleeping.

I would talk to the nanny and let her know that you don't mind if she talks briefly while the child is awake, but you would prefer it if she uses his nap times to hold her conversations since you would prefer her to spend his awake hours concentrating on him. After all, that is what you are paying her for, right?

She should understand because what you are asking is completely reasonable in my opinion.

I'm sure this has been asked before, but how much is too much? I've been home the last week with our PT nanny and noticing that she's on the phone constantly. The conversations aren't about anything urgent, just normal chatting and gossip. Don't get me wrong, I know it's boring being home with the kids since I do it myself during the week too so of course I'm not saying she shouldn't take the occasional call or text. There's a fair amount of downtime during DC's nap though so is it reasonable to limit longer coversations to then? She doesn't do any cooking or housework so that's not taking up her time during naps. Today I heard her upstairs talking for a long time while our DD was alone in the basement. She's a young toddler so I leave her alone for a minute or so sometimes to run up to the kitchen or bathroom, but I wouldn't leave her for a long time to go take a call. What's the best way to approach this?