What if they want to contact you and you use Signal?

IME you say “Sorry I only use Signal” and either they change or you don’t get in contact with that person.

If you change and abandon your principles were they really principles in the first place?

If you really really really need to? You use Whatsapp.

If you don't need to? You tell them to get Signal.

Not GP - I tolerate Whatsapp but I draw the line at SMS.

RCS?

You call them. Or use SMS. Or use e-mail.