"While Encrypting Mobile Field of Contact"
We use SOQL query while sending out messages.If you query encrypted data, invalid strings return an INVALID_FIELD error instead of the expected MALFORMED_QUERY.

As per salesforce restriction, Encrypted fields can’t be used with the following SOQL and SOSL clauses and functions:

  1. Aggregate functions such as MAX(), MIN(), and COUNT_DISTINCT()
  2. WHERE clause
  3. GROUP BY clause
  4. ORDER BY clause

So to be able to send out messages from Contact, we suggest you to do not encrypt Mobile field.