IVR (DTMF codes) on iPhone 16 with iOS 18 not working on incoming calls(?)

Hi -

I've definitely used my iPhone 16+iOS 18 to traverse 'phone trees' as I contact customer support/etc at a company. This is always on an outgoing call, and near the start of the call (other end answers, asks me to "press 1", etc)

Yesterday, I had problems with trying to enter DTMF codes (to enter my credit card number), this time it was on an INCOMING call (I had contacted Singapore Airlines on their website chat, they called me back on my iPhone, and part of the process included entering my CC # into their automated IVR system). We tried MANY times, but it seemed their end never heard DTMF from my iPhone.

The last time I tried, I noticed that the "keypad" screen on my iPhone was acting more like a "dialer" screen: as I typed digits, it was putting "()" around numbers (like for area code). I also noticed a GREEN "call" button at the bottom (usually, when it's just a DTMF keypad, it would only have the RED hangup button at the bottom).

So, I think the iPhone thought I wanted to start a 3-way call, and was letting me dial another number to add them (even though all I did was hit "keypad" and start typing digits).

I finally gave up on the iPhone and went back to an old-school "landline" phone to complete the process. It worked fine (so it wasn't a problem with Singapore Airlines' IVR system).

Is there any way for me to tell the iPhone "I want to enter DTMF codes instead of dialing a new number"? (assuming that it what was happening)

Thanks in advance for any assistance.

EDIT: I thought I'd add: this is stock iOS 18.0 (an updated version that downloaded after I received my iPhone 16) - not one of the new betas