Q: iMessage while on a phone call

iMessage uses the data channel, not voice.

If you are on a GSM network(ATT,T-mobile) being on a phone call does not matter.

Verizon CDMA network might cause problems since it doesn't do voice and data at the same time.

Don't know about Verizon's LTE network.

I'm on Verizon LTE and I can confirm that I don't receive iMessages while on the phone, they are held until I hang up.  Perhaps if the sender has the option "Send as SMS" enabled it might be converted to SMS, but even if that's true it is hard to control what everyone else does with their iPhone settings.

Check this article.  Especially note what type of cellular network you are on.  CDMA networks (e.g., Verizon in the U.S.) do not support simultaneous voice and cellular data.

iOS: Understanding cellular data networks

Same for LTE

cdr13180 wrote:

 

I get text messages though while on the phone from non iPhone users... just not from iPhone users.

Because texts from non-iPhone users do not use your cellular data service, but iMessage does.  CDMA does not support simultaneous voice and data, but it will do simultaneous voice and SMS/MMS.