I'm thinking of keeping my US phone on T-Mobile and getting another phone and number in the Philippines. What do you think?

Can't I just put an eSIM on my phone and use two numbers on one phone? That's what I did.

I've been using it this way for over 10 years. My T-Mobile phone stays at home on wifi, and my own phone only uses wifi calling.

Go for it. I did the same. I used Globe as a second eSIM, then switched my Philippine number to primary and kept T-Mobile as secondary. But there's something to watch out for. If T-Mobile finds out you've been abroad for a long time, they shut down the line. I think it's around three months, but I'm not sure exactly. So if you turn off the T-Mobile line for a few days in the settings and turn it back on about once a month, the countdown resets. I've been doing this for 3 years with no problems. Also, you can keep both lines on at the same time, so you won't miss calls or texts. When you need to contact people in the US, use FaceTime calls or texts, Facebook Messenger, or video calls. If you call directly, it's $2 to $3 a minute.

I keep AT&T and use an international calling plan. It's $10 a day on days I use it (I use it every day). But there's a $100 cap, so it works out to about $3.33 a day. T-Mobile charges $0.25 a minute, which is huge. 60 minutes × $0.25 is $15 an hour. T-Mobile is cheaper in the US, but in the Philippines it's ridiculously expensive. For calls within the Philippines I use my girlfriend's phone, I use WhatsApp for friends in Vietnam and a few other countries, and I listed that number as my phone number on my immigration forms.