The modern form of 信 consists of “words” 言 and “person” 亻/人, the latter lending its sound, and possibly the meaning “say”. In earlier forms instead of person acting as phonetic, it had “thousand” 千 (which itself uses 人 as phonetic) and still earlier “heart/mind” 心.¹ The earliest form might have been taken as “someone who speaks from the heart” is believable, trustworthy. The meaning of communication may have originated in a trusted envoy.²
A longer article on 信 is here.
信仰 shinkō faith; belief
信頼 shinrai trust; confidence
送信 sōshin transmission; sending