There is no such thing on iPhone
Settings > Messages > iMessage [Enable]
This will be enabled automatically by your cellular service provider if your have a data plan. Read this..
iMessage is carrier specific service. You need to have data plan loaded on to your plan. Contact them and ask of internet plan and then activate.
"Waiting for activation........" :- waiting for carrier's server to respond, Apple has no role to play in this waiting.
Set up cellular service on iPhone
The cellular connection on your iPhone requires a SIM from a carrier; contact your carrier to set up a cellular plan.
iPhone X and earlier models support one cellular plan using a nano-SIM.
iPhone XS, iPhone XS Max, and iPhone XR support Dual SIM using a nano-SIM and an eSIM (not available in all regions; iOS 12.1 or later).
iMessage and FaceTime uses internet data. If your network is bad the then you may have difficulty is sending iMessage and call on Face Time
It is always better to use Wifi during FaceTime
Once the iMessage is activated then FaceTime will automatically get activated
Set up iMessage and SMS/MMS on iPhone - Apple Support
https://support.apple.com/en-in/guide/iphone/set-up-imessage-and-smsmms.../ios